Well, I have the grouped items down pat...and I put some thought into the trade/sell thing...if you can sell your items back...why would you need trade?
Well I would have thought you could sell basic items back to the shop for a lower price, but there may be some items that are rare, and sought after(this is moving a bit down the line from stealing credits, or sticky topic, but could still apply). If certain items were only in teh shop, 1 or 2 at a time, then they may be in greater demand, and so, the person who buys them, might be able to sell them for a higher price.
It's all about supply and demand