The best place to ask about a gun's particular value is in one of the forums that we already have.
For shotguns, we have a shotgun forum where quite a few very knowledgable people hang out. Ditto for rifles, handguns, and blackpowder firearms.
There's really no need for a specific valuation forum when we have all of the others, but thank you for the suggestion, and welcome to TFL.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.