I recently thought I'd try out an online bookswapping site, Bookmooch. You sign up; you list some books you'd like to get rid of, for each of which you get 1/10th of a point; if someone asks for one and you send it you get 1 or 3 points, depending on whether they're in your country or another; if you mooch a book you pay 1 or 2 points, depending on whether it's a national or international mooch.
Bookmooch has been fretting over the free rider problem: some people sign up, mooch lots of books and are never heard of again. Quite why this came as a surprise is anybody's guess. On listing my books and getting requests for them, I found that the postage was on my nickel - in other words, having paid for the books once, I must then pay again to give them away. It's true that if I sent them internationally I get 3 points - in other words, I can get 3 books locally for every book I give away - but the last thing I want is to end up with three times as many books as I started with. I'm trying to FREE UP shelfspace.
In other words, other things being equal I would happily be adding more books to the system than I took out of it. On my last trip to London I hauled a whole suitcaseful of books to the Idea Store formally known as the local library. I don't really want to replace them with competitors for restricted shelfspace; at the risk of stating the obvious, releasing them into the Bookmooch pool, when postage for 50+ books had to be paid by me, was not in the realm of the possible. So Bookmooch, bless it, is selecting for free riders and selecting against altruists because they're not altruistic enough. In rewarding people for sending books internationally it's also doing its bit to promote global warming.
Having said all that, it would be possible to use the system to encourage local exchange of books. It's possible to browse books by location - in the US not just by city but even by ZIP code, in the UK by city, in France by department, in Rest of World by country. It would be perfectly possible to encourage people locally to sign up, and to make a point in 2008 of looking first for the books one wanted locally. It wouldn't help authors pay the rent, no, but it would help to reduce one's carbon footprint. From that point of view it's A Good Thing.