Right.  He is more than welcome to sell his gift.  But first he needs to pay his Customs charges due and have the item in his posession (rather than saying that he is refusing it, sending it back to us, and that we are somehow responsible for this auction - which we are not)

He can take part of the money earned from the auction and pay Customs, if that's what he'd like to do.  And then he'll be in posession of the item and can ship it himself directly to the winner.