of course they can, otherwise anybody could offer just 1 cent and hold up the possible sale of the item at the buy-now price until the seller responds to the offer (if they do not have auto-accept/decline settings). That would be an untenable situation for a seller, and buyers can not expect that a listing would effectively be put on hold until their offer is dealt with..
If the Buy-Now price is a fair price I recommend buying it . . . . . . . as some sellers, self included, will let an offer sit for over 24hours so that someone watching the item may actually use the BIN out of fear that any current offer will be accepted.
