This place is really made for the collector. Tons of rare LPs and out-of-print CDs that you can't find anywhere else. This is also the place to go for lesser known bands or local artists that you'd have a hard time finding otherwise.
I've had both bad and good experiences with the clerks. Sometimes they can't be bothered with you (one woman a few years ago was on the phone the entire time we were in there and didn't even acknowledge our presence) but then the young man that was in there last night was super nice and not pretentious at all.
I've never traded (I always use Zia for that) so I don't know what kind of money you get but I'm sure you'd get a lot more for rare stuff. Don't bring your Britney or Kanye here.
This is the place to go for music lovers!