Mike,
Older Huntsmans may or may not be considered a firearm, if it can be fitted with centerfire barrels, it is a firearm and must be sold in accordance with ATF laws just like any other firearm, I would consider it a firearm as opposed to a muzzleloader only until you learn otherwise. As for selling on the internet, it's done all the time, but the transfer of the firearm to the new owner must be done thru a dealer if it isn't done face to face with a resident of your state, IF you learn it is a firearm. If it's a muzzleloader only, it may be transferred directly to the new owner providing their state doesn't preclude it. They're listed and sold here in the classifieds all the time, but buying/selling/trading isn't allowed in any discussion forum here on GBO.
Tim