Lua Question on NPC System: Buying things

[Fermantor]

Hello everyone. I have a few questions about the npc question for the trade function.
Things I know how to do, but wanna do "better" are:
-Send trade window only if specific storage is on X
-Trade more items, the more storageValue you got
-Trade only on special word (not trade)
-Buy with Backpacks enabled

What I want:
Add items the usual way with the "shopModule:addBuyableItem" function and only check in the trade request for a storage.

Here is a npc script, that has a few of this things, but (imo) very poorly done

local t = {
     [18457] = {price = 150, price2 = 0}, -- [ITEMID TO SELL] = {Buy cost (0 = not buyable), sell cost (0 = not sellable)}
     }
     
local onBuy = function(cid, item, subType, amount, ignoreCap, inBackpacks)
    local price = t[item].price*amount
    if inBackpacks then
        price = price + 20
    end
    if getPlayerMoney(cid) < price then
        selfSay("Es tut mir leid, aber Du hast nicht genug Geld.", cid)
    else
        if inBackpacks then
            local Backpack = doCreateItemEx(1988, 1)
            doAddContainerItem(Backpack, item, amount)
            doPlayerAddItemEx(cid, Backpack)
        else
            doPlayerAddItem(cid, item, amount)
        end
        doPlayerRemoveMoney(cid, price)
        doPlayerSendTextMessage(cid, 20, "Du hast " .. amount .. "x " .. getItemName(item) .. " für " .. price .. " Gold gekauft.")
    end
    return true
end

local keywordHandler = KeywordHandler:new()
local npcHandler = NpcHandler:new(keywordHandler)
NpcSystem.parseParameters(npcHandler)
local talkState = {}

function onCreatureAppear(cid)                          npcHandler:onCreatureAppear(cid)                        end
function onCreatureDisappear(cid)                        npcHandler:onCreatureDisappear(cid)                     end
function onCreatureSay(cid, type, msg)          npcHandler:onCreatureSay(cid, type, msg)        end
function onThink()                                                      npcHandler:onThink()                                            end


function creatureSayCallback(cid, type, msg)
    if(not npcHandler:isFocused(cid)) then
        if msg == "hi" or msg == "hallo" or msg == "hello" then
            if getPlayerStorageValue(cid, 51234) == 2 then
                npcHandler:say("Oh, hallo! Ich bin der Gnome-Mensch Beziehungs Assistent. Ich tausche Deine {Tokens} gegen {Equipment}, versoge Dich mit Missions {Items} und rede mit Dir über Deine {Beziehung} zu uns Gnomen! Außerdem tausche ich Teile von {Statuen} ein.", cid)
                npcHandler:addFocus(cid)
            else
                npcHandler:say("Du scheinst neu hier zu sein. Sprich am besten erstmal mit Gnomette!", cid)
            end
        else
            return false
        end
    end
    local talkUser = NPCHANDLER_CONVBEHAVIOR == CONVERSATION_DEFAULT and 0 or cid
    local shopWindow = {}
    if msg == "job" or msg == "beruf" then
        npcHandler:say('',cid)
    elseif msg == "items" then
        if getPlayerStorageValue(cid, 51234) == 2 then
            for var, ret in pairs(t) do
                table.insert(shopWindow, {id = var, subType = 0, buy = ret.price, sell = ret.price2, name = getItemName(var)})
            end
            openShopWindow(cid, shopWindow, onBuy, onSell) 
            npcHandler:say('Hier ist mein Angebot. Es ist nicht groß, aber es erfüllt seinen Zweck.', cid)
        end
    elseif msgcontains(msg, "bye") then
        npcHandler:say("Bye.", cid)
        npcHandler:releaseFocus(cid)
    end
   
    return true
end

npcHandler:setCallback(CALLBACK_MESSAGE_DEFAULT, creatureSayCallback)