We use the blue book, agree on the percentage condition, and then, if the gun is something we think we can sell, usually offer about 80% of the blue book agreed upon value. We don't trade for Hi Points, Davis, Lorcin, or any of the "watch list" guns, and if the gun is in horrible condition, we politely decline. Some just want rid of the things and make us ridiculously low offers which we sometimes take. Right now, things are slow so we often turn down some things that would be sellers at a better time.
A used Gen 1 Glock will bring 350 in 95% condition, later gens a little higher. Examples in the past week: Remington550-1, 90% $100, sold for $150 later. Security Six 4", 98% with box and papers got the owner $300, sold for $379. We'll take a little less and go for return customers.