You don't have to invent the one app that enables people all over the world to share their stuff. Apps like these exist and they're not really popular yet as far as I know. Not enough people want to take part in such a system and a critical mass is required for it to work.
One reason might be that sharing useful items doesn't need an app. Yet the internet helps. You can easily find people who might be interested in sharing tools like a drilling machine for example. There are various forums, but why not use facebook? I think it's horrible when it comes to gathering private data, but to use such an excellent infrastructure for a noble purpose can't be bad. Even if facebook should turn out to be the place where bartering, sharing and other forms of exchange happen in the future - it's better than not seeing it happen at all. Furthermore I believe that every online community with a great number of users would collect data once it's grown big enough. We need to accept that and focus on how to use it for the benefit of mankind. It's being used anyway.
Still a better way is to use another kind of social network - your friends. What about having a clothes exchange party at your place? It's one of the coolest events I can think of. Everybody gets new clothes and rid of the old ones. But their not thrown away, but actually reused. And it's all for free.
You're not going to get rich by doing that. You're not going to be famous and win the Nobel Peace Prize. But you're having fun and acting sustainably. That's it.