In the past, whenever a user lost an item in a Steam trade scam, Valve would get it back for you. Now, Valve's policy has changed and it will no longer help victims of Steam trade scams recover their lost items.
"We sympathise 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," said Valve. "All trade scams can be avoided."
"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."
Valve advices users to read its Q&A page, which features examples of typical trade scams, in order to help users avoid these kind of situations.