Hey,
Firstly, what you do in single player is really your business. As long as it doesn't effect anyone else, then not many people have a problem with it (just don't go telling people either, btw). That said, as far a Single Player trading is concerned, what you just described isn't accepted.
You got to understand, that when you trade something off, you lose the item. There's no such thing as "I want to trade this off, and keep it too". You either have it, or you don't. The economy would be pretty shot if people allowed what you just described, because what's to stop you from trading off the same item as many times as you need.