IMO that makes no difference. Either the seller shares the ROM with the community and takes the risk of getting less money or the buyer shares the ROM and takes the risk of devaluating the cart. So if the buyer agrees with releasing the cart, he will most likely bid less. Therefore most likely the cart will go to somebody who won't share.
He really would like to share the game with the community, but he's going to leave that decision up to the buyer.
So in fact it is up to the seller (and not the buyer) to decide.