Bluefly is overpriced in general, I find, and frequently very overpriced. I rarely see what I consider real bargains there. I've even received discount coupons and gone looking for something to buy, only to come up empty-handed. Maybe those into designer labels would have more luck.
Sierra Trading Post, however, is the real deal. It's easily my favorite online clothing discounter. Yoox is more fashion-foward, but they sometimes have nice bargains. I also like browsing the sale section at Ben Silver, though I rarely buy. Some good shoe sales.
On eBay, Grapevine Hill gets in some quality shoes and has a very good reputation. And if you're interested in vintage tweeds and other country clothing, Bookster1UK often has interesting items, if you're willing to pay the shipping from England.
In terms of non-discount sites, Franco's is a favorite, though more for shoes than other items. The site is a bit hard to navigate, but I like those guys, so I'm willing to cut them some slack. And their site is still better than what Saks and Neiman have managed to put together.