Pros Closet is reputable and has been around for a long time -- a decade maybe? If not longer? I've never thought of their prices as being low per se, but rather they are in what I would term "The Reasonable Zone". I've probably bought from them in the past, but I don't remember specifically.

BTW, The Pros Closet once got a prominent mention by President Obama:

Obama mentions The Pro's Closet founder in letter on TPP | Bicycle Retailer and Industry News

How cool is that?