I liked the Avalon Exchange better back when it was located in Oakland, but the new location in Squirrel Hill isn't bad! It's smaller, of course, but their selection is generally pretty good quality and they have a variety of sizes and styles. However, I think it's a misnomer to call it a vintage store; there are a few vintage pieces here and there, but it's mostly modern pre-owned clothing. Also, if I don't find clothes that I like, there are always shoes or some jewelry that catch my eye!
My reason for the 3 star rating is that the staff tends to be pretty self-involved and pretentious, and unwilling to help you unless you're a hipster fashionista like them. They also tend to be super picky about what clothing they'll actually exchange, so I never know what they'll take in of mine.