Ok your asking 2 questions:
1) The answer to your "But it doesnt? o.o it just adds the item to the vendor and charges gold for it :/"
you going to use this command to add the item to the vendor:
Code:
.npc additem <ITEM ID> <AMOUNT> <EXTENDED COSTS>
by using it in this form:
Code:
.npc additem <ITEM ID> 1 3900
Second answer to "
It is taking the items out of my bag to trade in BUT its not displaying that i need X ammount of X to buy this item... Any fix?"
You'll need to make a custom patch for this and put your new custom .dbc in that MPQ which your players will need to put in their directories.