A friend wishes to research a rifle he has. It was probably made around 1790. A second lock (percussion) for it was made sometime later, perhaps in the 1840-50s. Most of the rifle is original, but a few parts (forward sling swivel, patch remover on the ramrod, aperture sight) are not. The original rifle had a peepsight but it was lost and a replacement was made for it. A set screw controls its elevation. See photos of the gun, its major parts and proof marks.
We were not sure what the bullet-like item peeping out of the buttstock is. I thought it to be a guide for making bullet molds but the owner says it is smaller than the nominal .63" caliber of the barrel. The gun is operable and fires patched .58" balls with reasonable accuracy.
He asked me to help him sell this gun. How would I go about figuring out its worth and finding a buyer?