Flayderman says that Union Firearms Co. was a trade name used by W.W. Marston. Probably dates from the 1850's but might have been sold a little later, no particular connection to the civil war unless you know of something. CW soldiers did sometimes carry something like that as a "backup" if they purchased it privately, but revolvers were more desireable for that purpose and were available at the time. Would be worth at least several hundred dollars but condition is everything when negotiating a price on those kind of arms. Not gonna be some 1000$ rarity I don't think, unless it was pristine in a case or something. Being a muzzleloading antique of questionable functionality, I don't see any restrictions for the sale of it to anyone who wants to pay the price. Sell it wherever they will let you list it, and don't worry about paperwork.
Your gun is like your nose, it is just wrong for someone else to pick it for you!