Building relationships with these exclusive brands is virtually impossible for startups. These are multi-billion dollar luxury brands that have no desire to work with a piddly startup. You can get thousands of non-audience driving mid-grade or commercial brands, but for exclusive brands, yah you need special skillz...
One that comes to mind is Nasty Gal. I think it's missing the problem of men; we have no equivalent and finding good clothes can be a challenge.
Maybe like Wanelo, Polyvore, Pinterest, Etsy, Fab, Fancy etc
It would set me back like $700 to buy a pair of well cut pants, used. These pants have legs that curve when hanged which translate to aesthetic looking stacks when worn. You can reverse engineer these things in your room - or pay people to do so - with quality fabric and you'd be looking at very nice margins, selling them for a fraction of the competition. Women have plenty of options like this whereas men do not.
All in all though I think this is a terrible thing to ask HN. If you browse enough of these clothes you get a sense of envy; you'd easily want to start paying unreasonable prices for things you don't need. I've been browsing fancy (and subsequent googling) for the last hour+ since I've read your response.
It's a good industry to get into but I think you should have some sense of passion driving you if you expect to differentiate yourself from the competition.