Code:
# Buy-Only and Sell-Only Shops 
# by RPG Advocate
#==============================================================================
# ** Game_Temp
#------------------------------------------------------------------------------
#  This class handles temporary data that is not included with save data.
#  Refer to "$game_temp" for the instance of this class.
#==============================================================================
class Game_Temp
  #--------------------------------------------------------------------------
  # * Public Instance Variables
  #--------------------------------------------------------------------------
  attr_accessor :shop_type                # map music (for battle memory)
  #--------------------------------------------------------------------------
  # * Object Initialization
  #--------------------------------------------------------------------------
  alias boasos_init initialize
  def initialize
    boasos_init
    @shop_type = 0
  end
end
#==============================================================================
# ** Window_ShopCommand
#------------------------------------------------------------------------------
#  This window is used to choose your business on the shop screen.
#==============================================================================
class Window_ShopCommand < Window_Selectable
  #--------------------------------------------------------------------------
  # * Object Initialization
  #--------------------------------------------------------------------------
  def initialize
    super(0, 64, 480, 64)
    self.contents = Bitmap.new(width - 32, height - 32)
    @item_max = 3
    @column_max = 3
    @commands = ["Buy", "Sell", "Exit"]
    self.contents.font.name = "Arial"
    self.contents.font.size = 24
    refresh
    self.index = 0
  end
  #--------------------------------------------------------------------------
  # * Refresh
  #--------------------------------------------------------------------------
  def refresh
    self.contents.clear
    if $game_temp.shop_type == 0
      for i in 0...@item_max
      draw_item(i)
      end
    end
    if $game_temp.shop_type == 1
      self.contents.draw_text(4, 0, 324, 32, "You can only buy at this shop.")
      self.index = -1
      update_cursor_rect
    end
    if $game_temp.shop_type == 2
      self.contents.draw_text(4, 0, 324, 32, "You can only sell at this shop.")
      self.index = -1
      update_cursor_rect
    end
  end
  #--------------------------------------------------------------------------
  # * Draw Item
  #     index : item number
  #--------------------------------------------------------------------------
  def draw_item(index)
    x = 4 + index * 160
    self.contents.draw_text(x, 0, 128, 32, @commands[index])
  end
  #--------------------------------------------------------------------------
  # * Cursor Rectangle Update
  #--------------------------------------------------------------------------
  def update_cursor_rect
    if $game_temp.shop_type == 0
      super
    end
    if $game_temp.shop_type == 1 || $game_temp.shop_type == 2
      self.cursor_rect.empty
    end
  end
end
#==============================================================================
# ** Scene_Shop
#------------------------------------------------------------------------------
#  This class performs shop screen processing.
#==============================================================================
class Scene_Shop
  #--------------------------------------------------------------------------
  # * Main Processing
  #--------------------------------------------------------------------------
  def main
    @help_window = Window_Help.new
    @command_window = Window_ShopCommand.new
    @gold_window = Window_Gold.new
    @gold_window.x = 480
    @gold_window.y = 64
    @dummy_window = Window_Base.new(0, 128, 640, 352)
    @buy_window = Window_ShopBuy.new($game_temp.shop_goods)
    @buy_window.active = false
    @buy_window.visible = false
    @buy_window.help_window = @help_window
    @sell_window = Window_ShopSell.new
    @sell_window.active = false
    @sell_window.visible = false
    @sell_window.help_window = @help_window
    @number_window = Window_ShopNumber.new
    @number_window.active = false
    @number_window.visible = false
    @status_window = Window_ShopStatus.new
    @status_window.visible = false
    if $game_temp.shop_type == 1
      @command_window.index = 0
      @command_window.active = false
      @dummy_window.visible = false
      @buy_window.active = true
      @buy_window.visible = true
      @buy_window.refresh
      @status_window.visible = true
    end
    if $game_temp.shop_type == 2
      @command_window.index = 1
      @command_window.active = false
      @dummy_window.visible = false
      @sell_window.active = true
      @sell_window.visible = true
      @sell_window.refresh
    end
    Graphics.transition
    loop do
      Graphics.update
      Input.update
      update
      if $scene != self
        break
      end
    end
    Graphics.freeze
    @status_window.item = nil
    @status_window.refresh
    @help_window.dispose
    @command_window.dispose
    @gold_window.dispose
    @dummy_window.dispose
    @buy_window.dispose
    @sell_window.dispose
    @number_window.dispose
    @status_window.dispose
    $game_temp.shop_type = 0
  end
  #--------------------------------------------------------------------------
  # * Frame Update
  #--------------------------------------------------------------------------
  def update
    @help_window.update
    @command_window.update
    @gold_window.update
    @dummy_window.update
    @buy_window.update
    @sell_window.update
    @number_window.update
    @status_window.update
    if @command_window.active
      update_command
      return
    end
    if @buy_window.active
      update_buy
      return
    end
    if @sell_window.active
      update_sell
      return
    end
    if @number_window.active
      update_number
      return
    end
  end
  #--------------------------------------------------------------------------
  # * Frame Update (when command window is active)
  #--------------------------------------------------------------------------
  def update_command
    if Input.trigger?(Input::B)
      $game_system.se_play($data_system.cancel_se)
      $scene = Scene_Map.new
      return
    end
    if Input.trigger?(Input::C)
      case @command_window.index
      when 0
        $game_system.se_play($data_system.decision_se)
        @command_window.active = false
        @dummy_window.visible = false
        @buy_window.active = true
        @buy_window.visible = true
        @buy_window.refresh
        @status_window.visible = true
      when 1
        $game_system.se_play($data_system.decision_se)
        @command_window.active = false
        @dummy_window.visible = false
        @sell_window.active = true
        @sell_window.visible = true
        @sell_window.refresh
      when 2
        $game_system.se_play($data_system.decision_se)
        $scene = Scene_Map.new
      end
      return
    end
  end
  #--------------------------------------------------------------------------
  # * Frame Update (when buy window is active)
  #--------------------------------------------------------------------------
  def update_buy
    @status_window.item = @buy_window.item
    if Input.trigger?(Input::B)
      if $game_temp.shop_type == 1
         $game_system.se_play($data_system.cancel_se)
         $scene = Scene_Map.new
      else
      $game_system.se_play($data_system.cancel_se)
      @command_window.active = true
      @dummy_window.visible = true
      @buy_window.active = false
      @buy_window.visible = false
      @status_window.visible = false
      @status_window.item = nil
      @help_window.set_text("")
      end
      return
    end
    if Input.trigger?(Input::C)
      @item = @buy_window.item
      if @item == nil or @item.price > $game_party.gold
        $game_system.se_play($data_system.buzzer_se)
        return
      end
      case @item
      when RPG::Item
        number = $game_party.item_number(@item.id)
      when RPG::Weapon
        number = $game_party.weapon_number(@item.id)
      when RPG::Armor
        number = $game_party.armor_number(@item.id)
      end
      if number == 99
        $game_system.se_play($data_system.buzzer_se)
        return
      end
      $game_system.se_play($data_system.decision_se)
      max = @item.price == 0 ? 99 : $game_party.gold / @item.price
      max = [max, 99 - number].min
      @buy_window.active = false
      @buy_window.visible = false
      @number_window.set(@item, max, @item.price)
      @number_window.active = true
      @number_window.visible = true
    end
  end
  #--------------------------------------------------------------------------
  # * Frame Update (when sell window is active)
  #--------------------------------------------------------------------------
  def update_sell
    if Input.trigger?(Input::B)
      if $game_temp.shop_type == 2
         $game_system.se_play($data_system.cancel_se)
         $scene = Scene_Map.new
      else
      $game_system.se_play($data_system.cancel_se)
      @command_window.active = true
      @dummy_window.visible = true
      @sell_window.active = false
      @sell_window.visible = false
      @status_window.item = nil
      @help_window.set_text("")
      return
      end
    end
    if Input.trigger?(Input::C)
      @item = @sell_window.item
      @status_window.item = @item
      if @item == nil or @item.price == 0
        $game_system.se_play($data_system.buzzer_se)
        return
      end
      $game_system.se_play($data_system.decision_se)
      case @item
      when RPG::Item
        number = $game_party.item_number(@item.id)
      when RPG::Weapon
        number = $game_party.weapon_number(@item.id)
      when RPG::Armor
        number = $game_party.armor_number(@item.id)
      end
      max = number
      @sell_window.active = false
      @sell_window.visible = false
      @number_window.set(@item, max, @item.price / 2)
      @number_window.active = true
      @number_window.visible = true
      @status_window.visible = true
    end
  end
  #--------------------------------------------------------------------------
  # * Frame Update (when quantity input window is active)
  #--------------------------------------------------------------------------
  def update_number
    if Input.trigger?(Input::B)
      $game_system.se_play($data_system.cancel_se)
      @number_window.active = false
      @number_window.visible = false
      case @command_window.index
      when 0
        @buy_window.active = true
        @buy_window.visible = true
      when 1
        @sell_window.active = true
        @sell_window.visible = true
        @status_window.visible = false
      end
      return
    end
    if Input.trigger?(Input::C)
      $game_system.se_play($data_system.shop_se)
      @number_window.active = false
      @number_window.visible = false
      case @command_window.index
      when 0
        $game_party.lose_gold(@number_window.number * @item.price)
        case @item
        when RPG::Item
          $game_party.gain_item(@item.id, @number_window.number)
        when RPG::Weapon
          $game_party.gain_weapon(@item.id, @number_window.number)
        when RPG::Armor
          $game_party.gain_armor(@item.id, @number_window.number)
        end
        @gold_window.refresh
        @buy_window.refresh
        @status_window.refresh
        @buy_window.active = true
        @buy_window.visible = true
      when 1
        $game_party.gain_gold(@number_window.number * (@item.price / 2))
        case @item
        when RPG::Item
          $game_party.lose_item(@item.id, @number_window.number)
        when RPG::Weapon
          $game_party.lose_weapon(@item.id, @number_window.number)
        when RPG::Armor
          $game_party.lose_armor(@item.id, @number_window.number)
        end
        @gold_window.refresh
        @sell_window.refresh
        @status_window.refresh
        @sell_window.active = true
        @sell_window.visible = true
        @status_window.visible = false
      end
      return
    end
  end
end