http://www.gameinformer.com/b/news/archive/2014/12/01/valve-changes-steam-gifting-rule-changes-to-combat-code-scamming.aspx

Valve has announced changes to its policies regarding game gifting that seek to protect the platform’s users. The move was announced last week by Valve employee Tony Paloma in the Steam Trading Cards Group.

Any game purchased as a gift that is placed in the purchaser’s inventory cannot be traded for 30 days. It can, however, still be gifted. 

“We've made this change to make trading gifts a better experience for those receiving the gifts,” Paloma writes. “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.”

It’s possible to trade gift games quickly after purchase, but before the payment method is determined to be invalid. The result of this scam is that the user on the wrong end of the trade loses both the trade collateral and the item(s) received.