First, what gauge? Assuming it’s a 12 ga, standard grade, which are far/away the most common, then in NRA grade (modern standards) Fair, it’s $250-400 for private party retail sale.
If you can sell it at all. Unfortunately, the appreciation of/demand for early/mid century, hammer pump guns is limited.
If a sub-gauge, better condition, or higher/special grade, then it could be significantly more. “Fairly good condition for its age” can mean a lot of things, so as usual the specific condition is important. A high grade 16 ga. could be worth thousands.
I still hunt occasionally with my great grandfather’s 1897, which he purchased new in 1903.
Grouse