Rare custom or limited run autocockers like that KAPP Flame are some of the paint guns that are really becoming collectible. A little patience and knowing the right way to present that gun in a BST thread and you should be able to get $425, possibly more if it's in great shape.
There are a lot of folks who would pay $300 for that gun, and a few who would pay $425 or more. Which is a better measure of value?
I could easily sell a mint condition 1964 mustang for $1500 - that doesn't mean it's not worth more.