I can't read anything on the lock plate, but it would probably have been made at one of the US armories, either Springfield or Harpers Ferry. The U is an inspector mark and I believe that the barrel band is from an M1863 as Johann Steele mentioned.
The method of conversion from flint lock to percussion is like I have seen on muskets converted by civilian gunsmiths. The drum looks to be rather crude and the hammer is a civilian style.
M1816 muskets were used during the Civil War by both sides, mostly converted to percussion but some still in the original flint lock. Whether this was used in the war is hard to say.
This musket was modified to be used as a shotgun by shortening the barrel and stock, and adding ramrod tubes. Many muskets and rifled muskets were modified in this manner by their owners with hacksaws and files. I have always liked the "Poor Boy Shotguns".
I would consider the price to be a bit high, but you should get other opinions on that.