who exactly do you "trust" though.
You make it sound like a very cut and dry thing. If you lmited the trading to only the close friends in your life, you'd never sell anything. How many of your close personal friends are buying hats / items on TF2 for hard cash ?
Valve need to implement some sort of procedure whereby they can rectify the actions of people who have hacked accounts. If you have received payment for an item that was stolen, then take that money back out of your account. If you have received items that were stolen, delete the item and given the OP his money back.
Then once you've done that, go after the guy that stole the item in the first place. You may not be able to get him to give his money back, but that's steams problem not the OPs.