In regards to flash sale sites: Gilt has a lot of house brands - you have to be careful about that because they say they are putting things at a discount, but they aren't sold anywhere outside of Gilt so...not sure it's a discount. However, I've gotten good deals on name brands. Same with Rue. But I have also had some bad experiences - you really really need to examine what you are buying - possibly looking on other sites to see what it really looks like. I've gotten some...weird things a few times.
I seem to prefer MyHabit - because it is Amazon and the shipping seems to be faster (and because you can often see a video of the clothing, which is AWESOME) - and Hautelook because it is Nordstrom and can be returned at Nordstrom Rack.
As for non flash sale sites, I'm not much help because my office is extremely casual (t-shirts, flip flops and jeans). I buy a lot of stuff on Revolve and Asos. Also Saks Off 5th, Neiman Marcus Last Call and Nordstrom Rack online.