I bought a Colt .38 revolver for $150, then about ten years later traded it to the same store for a $500 plate carrier with level 3A plates.

Turned out it should have been about $100 less due to the brand of plate carrier they were using, but whatever.
I have a buddy who has done well on gun trades/sales in the past, but it was with private sellers. Many of them knew what the price was supposed to be, but got tired of waiting for a buyer, or wanted what was being traded enough that the disparity in value was acceptable to them.