Avalon sells relatively high quality vintage clothes for reasonable prices. You're likely to find anything from paste-on mustaches and cheap aviators to brooks brothers' polos in good condition. The prices are higher than you might expect from a vintage store, but are about the same as what you'd pay on Ebay (even less for popular designers). I've found the staff to be helpful.
If you're looking to sell something, I would try Ebay first. Avalon buyers don't pay market price and are extremely picky.