Valve has taken direct aim at Steam code traders with its new policy for gift codes. While you can still give them away, you will no longer be able to trade Steam gift codes for thirty days after you receive them.
In a statement, Valve explains:
We’ve made this change to make trading gifts a better experience for those receiving the gifts. We’re hoping this lowers the number of people who trade for a game only to have the game revoked later due to issues with the purchaser’s payment method.
This will put a dampener on those people who buy cheap gift codes in bulk during sales and then try to sell them off (trade for cash) at a higher price. It probably won’t completely end the practise, but it’ll mean that the resellers have to invest their own money for longer.