Great pics of legs, feet, sandals and tattoos. Not a good picture of the gun.
From the little I can tell, it looks like one of the many foreign muskets imported during the American Civil War, perhaps Austrian, but without supporting evidence of CW use, it is just another old musket.
It doesn't appear to be any standard US musket, either original or conversion, but without better pictures it is hard to tell even what it is not.
When you can get good pictures of the lockplate, the top of the barrel and the front end, post them and maybe we can be of more help.
If it is a foreign musket, those are not bringing high prices; good ones go for around $400-600 depending on condition if complete and functional. Ones cut down for shotguns bring less.