IMO the fair price for a muzzleloader is geographical much more so than with modern firearms. In some areas of the country they are common as nails and just as cheap to buy, while in other areas quite scarce and so much more expensive. Supply and demand. So where a seller lives or bought the rifle from and where a buyer is plays a part in what each consider fair for the same rifle. IOW, a TC Hawken available for $200 in my neck of the woods just doesn't exist unless it is a worthless pile of junk.
I agree with bub, they wouldn't offer the 5 day inspection if they didn't think the rifle would match their description of it. The more honest and complete a description is in an ad, with good pictures, the less the chance of both parties not having the same opinions on the rifles condition. Communication, simple as that until both parties are on the same page, and have established a level of trust in each other. The seller puts in a lot of time and effort to prep an item, take pictures, research and write an ad to offer a saleable item for a fair price. A serious buyer should not be against also investing a little time as well. A simple phone call is a great tool to get to that point. More information will change hands than was written in the ad, a persons ethics can be easily learned from the call(s) and you just might meet a like minded person that will become a friend and not just somebody that you did business with once. IMO if a seller is honest with his description he doesn't need to offer the 5 day return, ever. If he wants to offer the return to back up that he is trying to be honest that's fine, but he doesn't also need to pay for the return if the rifle is as he said it was WHEN HE SHIPPED IT. There are just as many scam buyers as there are scam seller out there. Again, being on the same page BEFORE a commit to sell/buy will eliminate the problems later, or the need to return something. And with the trade count that Bill has provided here on GBO for sellers and buyers, you have one more tool to get a good feel for the other party. I have bought and sold on GBO and at many other places on-line for almost 20 years, and with almost zero problems anmd thousands of items. I have also never offered a return policy, and have never had anyone request it FWIW. And it was easy, I just did the above and earned the reputation of being an honest seller and buyer.