Can User Initiated Group Buying Work? Sam Altman thinks so.

I recently co-founded a startup called Grosper.com. The basic idea is that instead of building up a huge salesforce, why not let users initiate deals? This has been done by Loopt with their U-Deals product however, dont know how their doing though.<p>My startup calls it Make A Demand, and you're only allowed to do it for Real Estate and Travel.<p>But is there a genuine market here? Building owner are willing to give 20% discounts if three people decide to rent at the same time, that's pretty good.<p>But how would you co-ordinate the process? How would you make it simple? Easy? How would make the Dropbox of group buying so that's its not confusing?<p>Any thoughts?