A $400 giftcard! Usually when someone puts a $400 giftcard on an auction site its origins are, shall we say, QUESTIONABLE!
But, if things are square business, and you accepted check payment from an out-of-state stranger, then maybe you do not need to be internet auctioning. Seems like you're not 'auction-literate' enough to conduct that sort of business. At the very least you should've tucked the $400 check away for a week or two til delivery was confirmed.
On the other hand....if the GC was illbeggotten to begin with then you got what was coming.....OR, was the GC actually sent?
A number of things-that-make-you-say-hmmmmm about this...........
Now, IF the GC was legitimate in its origin you can contact the store where it was purchased, explain to them what happened, then they will most likely cancel out the balance (if it remains) and reissue you a replacement GC.
IF the GC has been used, they will be able to track it's use down to the store, register, transaction....then that store can check their video to see who was the user if that's the case.

