Good idea, as long as the original owner knows what's going on with their book (if the book is being borrowed, not sold.. ).

That may have to be decided on an individual basis - if there is a long waiting list for a specific book and it's being borrowed and the owner does not need it for some time, then it may be an idea to make a list of addresses and just pass it on like you said - the last person would mail it back to the original owner. It may cut some sort of cost down on shipping, each person who receives it would pay for the shipping to the next person.