The 33-year-old owner of a second hand shop in Rhyl was reported to police after offering the firearm on the social networking site.
Five firearms, including two shotguns, an air rifle, a bolt action shotgun and a revolver were found at his home and shop, along with some ammunition.
The shop-owner has since been released on police bail pending further inquiries.
Ch Insp Jason Devonport from North Wales Police said: "This is particularly concerning and thanks to the officers from Rhyl, for acting quickly by arresting and recovering these weapons and making North Wales safer."
A man trying to sell a shotgun on Facebook has been arrested.