Lua [NPC][TFS 1.x] Adding trade items after quest

[Jarofir]

My problem is that I want to make NPC trade with players only with specifik storage value (after finishing some quest) but my code is not working.

local function greetCallback(cid)
   if isPremium(cid) then
     npcHandler:say("you are premium", cid) -- only check
     if getPlayerStorageValue(cid,questStorage) >= 1 then
       doPlayerSendTextMessage(cid, MESSAGE_INFO_DESCR, "you have done quest") -- only check
       local shopModule = ShopModule:new()
       npcHandler:addModule(shopModule)
       shopModule:addBuyableItem({'brass helmet'}, 2460, 120, 1, 'brass helmet')
     end
     return true
   else
     npcHandler:say(getCreatureName(cid) .. ", I will speak only with premium players....", cid)
     return false
   end
end

No matter if the quest is done or not I still can buy this item. But the msgs "you have done quest" and "you are premium" pops up corectly. Any ideas?

Edit: I can see that the problem is that if anybody with quest started trade with npc, he will always have this item in his shop. How can i change that?