I heard the same complaint about scanning from a man who used to
offer prints from copy negatives from the ACF collection - he had a
100 page catalog of negatives.
The solution is simple - a policy of no returns.
I've always kept photos that I was unhappy with because I wanted the
seller to continue to do business with me. Ditto for merchandise
bought from basement hobby shop operators. At any time the seller
could cut me off.
Some of the guys selling prints (and Sunshine Models) are very slow
to respond. These sellers are not desparate for business.
I think Bob would make more money if he sold prints through the mail
in addition to the shows - there's very little additional expense to
what he has now - the cost of clerk to maintain a list, pick photos
and send them out. He doesn't bring all photos to all shows anyway
and many of his show customers limit their purchase to the amount of
money they have in their pocket.