can you look into a bit more? have they bid on other sellers too?
I can't recall the list of info at the mo, but does it show other bids to other sellers etc?
50% would be ok if they have bid on 4 shirts (for eg)and 2 just happen to be on the one seller.
feedback won't give you a clue as many people have multiple accounts, each with a different FB - and you can't bid on your own stuff with the same id as the one you are using to sell.