WELCOME TO THE FORUM!!! :thumbsup:
Hopefully, someone with greater knowledge than me will show up to answer your question.
In the meantime, I'll give a few thoughts about black powder muzzleloaders.
Very few guns which are (were) commercially made by a company are considered very valuable simply because there are no collectors of them.
By collectors, I mean the type of people who collect old historic firearms like Colts, Remingtons, Winchesters etc.
Most people who buy them intend to use them for something.
That something may be hunting, target shooting, re-enacting, hanging in a display case...
In the case of your GRRW gun for instance, I doubt that there is one person in the United States who has a Green River Rifle Works collection and would be willing to pay a large sum of money for your gun.
That is not to say there are no people who would want your rifle. I'm sure that many would but they would want it for its use, not for some collector value.
That said, a high quality factory muzzleloader can sell for anywhere from $400-$700. A custom made rifle in new condition can sell for $700-$3000 depending on the builder and the style of the rifle. I might add that with the devaluation of the dollar, some of the new Italian muzzleloaders are selling for $600-$900 but the majority of them are in the $350-$500 range.
Based on this, I think that if you want to use it for hunting then by all means do so.
Treat it like a high quality rifle but enjoy using it. After all, that is what it was made for.
zonie