This approach makes sense. I also think that sneakers (or streetwear in general) would make for a pretty compelling initial market. You'll obviously encounter the chicken-and-egg problem with this platform, so it would probably be good to try and hack one side of the market. In that light, definitely keep working together with the admins of these huge Facebook groups (I reckon you're in Basement, Supreme Talk, Palace Talk?!). These 40k+ groups make for an incredible opportunity to engage with your target audience. Hell, to get initial traction just raffle a pair of Yeezy's, Air Mags, or that new Gosha among the people that sign-up.

One thing I'd do differently though is start out with a website (vs. an app). It's very true that Facebook doesn't have any form of domain-specific search fields. But it's very accessible. I realize that n = 1 here, but the friction in having to download another separate app to my phone would be too high for me to switch from using the different Facebook groups. I think you'd want to make it as easy as possible to access your solution (or at least submit WTB requests) in order for this to take off. In that light, a website would be much better I think.

On another note, here's a really good read on why Facebook may help you out a lot, but would also most likely be your biggest competitor: http://alexmuir.com/facebook-is-the-new-excel

Anyway, very cool. Happy to chat more if you like -- I'm at aron@wanderlust.ly.