Do items in Buy Back disappear?
When selling items to merchants, the only items you are able to buy back are Uncommon (Green), Rare (Blue), and Very Rare (Purple).
After doing some testing, the answer to your question is yes - items will disappear from the "Buy Back" menu. You are only able to buy back the last 10 items you have sold to any merchant. There is no way to retrieve any items past the 10 limit.