Big W,
You did a great job with the photography. I'm not up to speed on market values, but I won't disagree with kje54.
Your rifle appears to be in very good condition overall, but I would not call it mint. I think I see a little rust freckling on the top flat of the barrel, and possibly a little pitting in the nipple seat. That's not a criticism, but just an observation.
Your rifle in .45 caliber is a little unusual, and I can't predict how that might affect your sale. Bigger bores seem more popular now, despite the fact that .45 caliber is more than adequate for deer. Also, "a lotta guys" gripe about the weight of blackpowder rifles, and your .45 will weigh more than a comparable rifle in .50 or 54 caliber. On the other hand, the relatively rarer .45 caliber rifle may have more collector value. I don't know.
The more knowledgeable potential buyers will probably want to see a photo of the bore, with good illumination. When or if you put this rifle up for sale, it will be important to include that.
It appears you are fairly new here. We are glad to have you! I don't know if you've browsed through any of the prior posts. If not, I would encourage you to go to the "Firearms For Sale" section of the classifieds and scan through the offerings there. I recall several Jonathan Browning Mountain Rifles (JBMRs) have sold on this forum over just the past few months. Read through those threads and you can get a better idea of what sellers were asking, what they settled for, and the types of questions potential customers will ask.
You have a nice rifle.
Best regards,
Notchy Bob