I could not find anything specific in the rulebook about what happens with discarded item cards. I assume you shuffle the item cards into the deck after they are sold/dropped. We had a situation where we had sold an item for gold, and when we purchased a new item, we got the same dang item.
Did we play that right? Or can we leave the sold item out until we are done with our shopping? (Don't get me started about buying random items, so weird.)