For those for whom it's a big issue, my suggestion posted in another thread (how many new threads do we need on this topic?) is to create a kind of secure exchange, where the organizer collects the prints and redistributes them. That way everyone who sends a print gets a print, and those who don't send prints get nothing. The downside is more shipping (and everyone would probably have to make a small contribution for mailing costs), but the attraction is more security, and it's less administration than the debt collection approach.
And for the rest of us, for whom the current exchanges seem to be working, we can continue participating in the existing exchanges, recognizing the possibility that return is not going to be 100%.