http://www.renttherunway.com/ has been doing something similar (high-fashion dress rentals) for a few years now. Of course, Rent The Runway is more centralized - it's more analogous to a hotel, while 99 Dresses is more AirBnB.
The fundamental idea of reselling used clothing is really cool and needs an overhaul from the usual Goodwill and other thrift stores, but 99dresses seems targeted at a specific audience that quite literally only wears something a couple times before tiring of it. I'm not sure how compatible that is with constantly changing fashions and questionable clothing quality from some of these clothing stores... I have a lot of old non-designer dresses I could be selling on this site, but after a couple times they've all got small problems (I don't want to sell something I mended!) or are out of fashion by a year or two.