I think that if the regulated price was either a ceiling that is low enough, or a floor which is high enough (or both, as in the case for wages), then harms would not result at all. A law which said "all organ selling must be either a free donation, or accompanied by a payment of at least $2 million" would probably be okay.

But that is a very noncentral description of organ selling. The majority of advocates of organ selling don't want such limits on it.