Avalon is a "buy, sell, trade" store for used and vintage clothing, shoes, accessories. It's mostly used contemporary clothing although there are some vintage things. You can purchase items, sell your things (and they'll pay you in cash 30% of retail value), or trade in your things for store credit (45% of retail value) to use right away or later (I believe store credit expires in a year).
I drop in a couple times a month or more, sometimes with a specific item(s) in mind that I'm looking for, and other times just to browse. Usually I'm successful (meaning that I purchase something :p) and on good days I manage to get quite a good deal by trading in something(s) first and then using the store credit right away. (Yesterday I traded in a pair of funky colorful flats and bought a green light jacket and black pointed flats... paid $11 something.) There are clothes in the back of the store that are half price (red tag) and sometime in the summer (I wanna say mid July? for the Squirrel Hill Sidewalk Sale), they have a $1 "Dollar Days" sale.
If you are thinking about selling or trading in your stuff, the best info/advice I can give is that they usually take name brand and unique/funky things... in great condition... and in season (fall/winter items now).