^^^^^^^^^^^^ I traded an older S&W 9 mm for a S&W SD9. They gave me more than I expected.
My suspicion is that even 2nd hand firearms are in demand. I was in howell's one other time looking for some accessories, I didn't really spend much time at the gun counter, but I also didn't notice a huge stock of used guns when I walked by it.
Go, take your Glock, if you are serious about a Windham "AR", talk to them. You may be surprised, and you can always walk out if you don't like the deal. If you have great will power that is.