Seconding Buffalo Exchange. Try not to go on a weekend. It helps if you dress trendy and put on some weird accessories when you sell. (Don't go in your gym clothes.)

It's nice that you're doing this for your friend, because you kinda have to have thick skin to sell your clothes at place like Buffalo. It's totally hit or miss. Sometimes they don't buy stuff that I think is right up their alley. Other times I see crap on their racks that is shrunken, pilly and ugly.