I see, well basically is a database issue.
You can add a vendor, going in the vendor table.
vendorGUID is the vendor model, itemGUID is the item wich the vendor sells, amount is how many items of that kind he sells.
Make a new row for every item and set amount greater to 1 only for items wich can be sold in stack, like arrows.
Now you need to spawn this vendor somewhere, so go in game as GM, go where you want the vendor and write this command: .npc spawn vendorID
Honestly i've never did it, so that's only theory.
You can try or wait for someone else to answer.
(if you try and your vendor is wrong, delete him... select him with your mouse and write .npc delete)