Vodkart

-- Callback onSell() function. If you wish, you can change certain Npc to use your onSell().
    function ShopModule:callbackOnSell(cid, itemid, subType, amount, ignoreCap, inBackpacks)
        local shopItem = nil
        for _, item in ipairs(self.npcHandler.shopItems) do
            if(item.id == itemid and item.subType == subType) then
                shopItem = item
                break
            end
        end

        if(shopItem == nil) then
            print('[Warning - ' .. getCreatureName(getNpcId()) .. '] NpcSystem:', 'ShopModule.onSell - Item not found on shopItems list')
            return false
        end

        if(shopItem.sell == -1) then
            print('[Warning - ' .. getCreatureName(getNpcId()) .. '] NpcSystem:', 'ShopModule.onSell - Attempt to sell an item which is only buyable')
            return false
        end

        local parseInfo = {
            [TAG_PLAYERNAME] = getPlayerName(cid),
            [TAG_ITEMCOUNT] = amount,
            [TAG_TOTALCOST] = amount * shopItem.sell,
            [TAG_ITEMNAME] = shopItem.name
        }

        if(subType < 1 or getItemInfo(itemid).stackable) then
            subType = -1
        end

        if getPlayerSlotItem(cid, CONST_SLOT_BACKPACK).itemid ~= 0 then
        local bp = getPlayerSlotItem(cid, CONST_SLOT_BACKPACK)
        local z = getContainerItemsById(bp, itemid)
        if #z >= amount then
            for i = 1, amount do
                doRemoveItem(z[i].uid)
            end
            local msg = self.npcHandler:getMessage(MESSAGE_SOLD)
            msg = self.npcHandler:parseMessage(msg, parseInfo)
            doPlayerSendTextMessage(cid, MESSAGE_INFO_DESCR, msg)
            doPlayerAddMoney(cid, amount * shopItem.sell)
            if(NPCHANDLER_CONVBEHAVIOR ~= CONVERSATION_DEFAULT) then
                self.npcHandler.talkStart[cid] = os.time()
            else
                self.npcHandler.talkStart = os.time()
            end
            return true
        else
            local msg = self.npcHandler:getMessage(MESSAGE_NEEDITEM)
            msg = self.npcHandler:parseMessage(msg, parseInfo)
            doPlayerSendCancel(cid, msg)
            if(NPCHANDLER_CONVBEHAVIOR ~= CONVERSATION_DEFAULT) then
                self.npcHandler.talkStart[cid] = os.time()
            else
                self.npcHandler.talkStart = os.time()
            end
            return false
        end
    else
        doPlayerSendCancel(cid, "I only buy items that are inside a BackPack")
        if(NPCHANDLER_CONVBEHAVIOR ~= CONVERSATION_DEFAULT) then
            self.npcHandler.talkStart[cid] = os.time()
        else
            self.npcHandler.talkStart = os.time()
        end
        return false
    end
end