If you take the item and click multiple times quickly when trading into the fence at the Thieves Guild you can sell it multiple times. Tested, and it works with all of the items you can fence. Not sure if this has be posted before but just an extra thing you can do.

I just checked this out. While you can give the item to Boris multiple times, you don't get paid for the item until it's removed from your inventory. The same thing can happen while giving items to the healer too.