What a cool concept. I have never sold anything, but I have bought stuff. Good Quality used items for a good price. There always does seem to be a line to sell items there, which means they are doing something right.
The staff is very knowledgeable about what the current trends are. They know what they are buying and what they are turning away. Don't feel bad if they don't take all of your stuff. Sometimes they have multiples or just don't think they can't sell the stuff. They aren't being rude, just doing there job. :D