I inherited an authentic blunderbuss and have no idea what it is worth
If it's in good condition, not cleaned up, I'll give you $250. Show me a photo.
Get it professionally appraised before selling. You may have a gold mine and not know it.
The problem is that many many many reproduction blunderbuss have been made. Lots and lots.
Identifying a repro from an original is a job for a real expert with the gun in his or her hands, not for over-the-internet.
If it's real, even in fairly poor condition, it's worth a lot. If it's a repro, then not.