It'll be interesting to see how this develops. I'm a little worried that people's first inclination will be to always offer half of everything and hope it goes through, which might cause the sellers to simply raise their prices on the site. Focusing on luxury goods is also a bit of a gamble because the people that buy they are generally savvy enough to watch for sales themselves (which often go far beyond 50% off).
That being said, this space desperately needs innovation and I hope I'm wrong about all of the points above!