Thanks for pointing me to the site paperbackswap.com. This is a really cool idea, specially how it manages book transfer and the concept of book credits. I think the middleman option is not very scalable and you should look at other options. The reputation concept suggested before does seem like a good idea. To make sure that a user is shipping high quality comics you may introduce the concept of deferred comic credits, until another user ratifies that this user indeed ships high quality comics. Good luck!!