Obviously the law would need to be tightened to prevent family's auctioning off body parts against the deceased's will.Um... You'd generally not be able to stop them, being dead or incapacitated.
The whole problem with organ donation (or indeed anything to do with belongings after death) is that frequently the deceased's wishes aren't respected.
Pretty easy to come up with a legal solution. 'The deceased's body parts may not be traded unless there's a solid contract with the deceased's signature filed with a relevant solicitor before the deceased's passing'.