That exchange app is very doable and a good idea. Counterfeiting could be stopped or drastically reduced by having a reputaiton system (like ebay or couchsurfing)

I think they should have that for travelers' stuff in general. I'm going to donate my Lonely Planet Eastern Europe to the next hostel I stay at, but I'd rather give it to somebody who could use it right away.

Craigslist is far too clunky for this sort of thing. I want something that lets you post things wanted and things people want to get rid of, matches them up based on GPS, and lets you IM them.