If you all think this is a cut and dry case you are completely wrong. Using the defence "how am I supposed to know it was stolen goods" does not hold up with physical items, nor does it hold up on steam with "digital" goods.
If the police found that you were repeatedly selling stolen good from a shop I'm sure just shrugging and saying "how am I supposed to know" would totally cut it.
I'm not saying there is a foolproof way or even a way to verify these people you are dealing with or the goods they are trading to you but that doesn't give you carte blanche to ignore Valve's warnings that this was happening and continue to trade in items of which you have no idea of the legality.
People are also seeing this from a consumer point of view, if he was just an innocent consumer who had been sent one or two dodgy games then yes this would be ridiculous. But to set up a business or service to trade these items in bulk is totally different. It is entirely normal for shops receiving goods from customers to take customer details.
I hope, assuming that you are legit, that you get this sorted but the only way I can see this working going ahead is if they set up some way to let you know what is illegal or not so you can possibly avoid trading with the same people more than once.
Further, please consider that in the same way you have alleged you have no way to know the items are illegal or stolen. How can Valve know that you are not deliberately dealing in these goods? The same defence you are using to get out of this the exact same reasoning for Valve to shut you down.
There is no malicious intent in this post or my first one, I am sorry this has happened to you, but as you felt the need to inform us of your plight I felt the need to present a counter argument and a bit of sense.