Here's what I would suggest:
1) Each member pays a small entry fee. This "Entry fee" is fully refundable when they leave the club, and is only there to ensure honesty.
2) The fee would be something small, maybe £3?
3) In exchange for the entry fee, the member receives a number of "credits". Each credit can be used to "buy" a single paperback book, and 2 credits will buy a hardback book.
4) A user builds up credits every time they send a book (and confirmation is received that the book arrived safely). They can then use these credits to "buy" a book from another member.
5) If a book does not arrive safely, the account of the sender will be deducted by an amount, maybe £1? When their account reaches zero, they cannot trade until they "top up" their account.
These are just initial ideas, and it may not work, but it needs to be done in such a way that its not a "reciprocal" exchange - i.e. you can have books from anybody, and not just somebody that you have sent a book to.
