I think people who post for sale items should justify their own prices if they put something up for sale. Like this:
Originally bought KSC Glock 19(Taiwan version) for $238
+ $30 for metal slide
- $10 for being used
- $30 for being fielded a few times
- $30 for leaky mag
Final price - $198
This way a potential buyer can see where all the prices are coming from and say something like "wait, how bad does the mag leak? If its real bad, how about -$50 for the gun?"
In addition, as sellers do this more often, the community can develop a general idea of how much "being used" should deduct off the final price. I can sort of picture a more formal system of selling guns being formed but it'll take some time to implement.
*edit*
I know SOME people do this, but just not enough
