Nothing. It happens.
I'm a trader over there (BostonCollegeFan) and have been trading for years, have a shit load of feedback and sometimes this happens.
A while ago on CAG I traded a decently old 360 game for Splinter Cell Double Agent when it first came out. The deal was going good and everything when the guy said "Oh shit bro I lost the manual to the game". Knowing that I hate not having manuals and didn't want to get into a huge feud (because I already sent my game to him), I just said "Its okay man, don't worry about it". Instead, three days later, he sent me a pm saying "Hey man, just went to bestbuy.com and bought you a brand new copy of Double Agent because of the missing manual thing. It should arrive at your house in a few days"
So the guy spent $120 on a trade for like a $40 game. Low and behold about a week later, a package from bestbuy.com shows up, brand new DA. People are crazy over there.