Valve won't help you with Steam trade scams
"Our community assigns an item a value that is at least partially determined by that item's scarcity. If more copies of the item are added to the economy through inventory rollbacks, the value of every other instance of that item would be reduced," say Steam's policy.
"We sympathize with people who fall victim to scams, but we provide enough information on our website and within our trading system to help users make good trading decisions."
Valve concludes that "all trade scams can be avoided".
So basically, they're saying 'it really is your own stupid fault if you get scammed, soz like'. Seems a bit odd to put the onus on the user, but there you go.
If you're going to trade, then you might want to do some reading up on scams first, so you don't get burned, because they're not going to help you out one bit