I concur with the Muzzleloading Forum. I look at it everyday for bargains and current market prices items sold.
Its a very educational multi-faceted forum covering each type of traditional muzzleloading firearm,, including shotgun (listed as "smoothbore") and significant historical periods, F & I, Rev War, War of 1812, etc.
I highly recommend this source for buying and selling muzzleloaders and accessories.