For some reason i can't open the first pic but i think sam is right, it does resemble a blunderbuss. Prices are all over on these types of guns and there are also many fakes and reproductions out there.
Blunderbuss, missing frizzen and lower jaw on the flintlock.
If it has a touch hole it used to be a working weapon.
As far as value, I would be surprised if it is worth much at all.
Look to see if it has any modern style screws or bolts in it, may help to let you know if it is really old.
Blunderbuss was popular in the 15th-16th century on ships for clearing the decks in a fight.
You could load that sucker up with anything. Shot, glass, rocks etc. Close range weapon.