Well, based on the pictures you sent me, you have an S.A. Goddard percussion shotgun "fowling piece", probably made in Birmingham, England between 1834 and 1856, though some were assembled later in the US from Goddard parts. The real English-made ones will have English proof house marks on the barrels and the American ones won't.
Yours, as closely as I can tell, is one of the plain grades and not the fancy engraved and silver damascened versions, but maybe it is just worn. I can't get good enough resolution to tell.
The highest price I've seen for a Goddard in the top "deluxe" inlayed and engraved grade was a bit under $750 at auction some time back.
Lower grade guns of that sort might sell now in the $200-$400 price range if they are clean and all parts work and are there, a lower price than a few years back. I can't tell from your photos what shape yours is really in, so i can't be very precise. If English and in good shape, people still shoot these, but if corroded from black powder and parts missing or hammers not functioning, it is just a pretty wall hanger.