Not able to trade item to another player

Long story short, I ran a GDKP a couple days back, killed saurfang... DBW drops, woohoo money for everyone. I was the masterlooter for the entire instance, and to encourage people to clear faster I told them to go and clear trash while the bidding was going on and I would trade the item to them before the next boss fight.

The winner of the DBW trades me a large sum of money, I attempt to trade the item to him and it says he is not eligible to receive it. I put in a petition and gave as much detail as I could, as a DBW isn't of much use to a priest. I kind of want to equip it just to see what stats it gives a priest when it procs, but I don't wanna accidentally disqualify a transfer. =p

My main question here is, what could have happened there and how can I prevent it in the future? DBW is a big-ticket item and I hate making people sit around a few minutes while my mod spams their screen with bidding stuff. From what people were saying in the raid is that sometimes the loot system will glitch if you enter the door while the chest is being opened, but I'm not entirely sure if I buy that. The guy was in the instance the entire run, was there for every boss fight and didn't afk or die or disconnect.