Here's what worked for me. If I had already traded items with a merchant previously, then the next time that I traded with that merchant, I would simply scroll to the very bottom of that merchant's inventory, and begin browsing their items from the bottom to the top or at least until I started seeing my old items appear in their inventory. However, if you start your browsing from the top of the merchant's inventory then you will probably see alot of the items that you sold to them previously.
