I used to list guns on GB for the store I managed. Consequently, I was constantly looking at similar guns to see what they sold for as a guide to listing. Listing on gun broker exposes your item to people in New York, Los Angeles, Chicago and other places where income levels dwarf those in less populated areas, so you could find stuff, but it wasn't always a good deal compared to the "local" price.
Look at the pictures closely... if they do not have more than 20 pictures of the item, including the bore and the area of the breech, ask them for better pictures. Ask questions you have BEFORE you place a bid. Look at the seller's feedback. If he has sold ten thousand items and nobody has given him bad feedback, you probably have a pretty honest seller. If it is described glowingly, like "bore excellent", "a few pits, but very strong rifling", "ultra-rare collector piece", etc. get suspicious. Get real suspicious. Used muzzleloaders NEVER have excellent bores. Beware of NIB listings, they might or might not be NIB.
If the buyer does not allow a reasonable inspection period, then don''t buy it. If you can drive to the place it is for sale, that is not a bad idea. Hands-on inspection is much better than looking at possibly doctored photographs. If you can call the seller and speak to someone who actually has the gun in his hands, that is time well spent. It is much easier to lie in print than over the phone because you can add tone of voice and hesitation on certain questions to your information.
Guns are all heavily inflated now. If you just gotta have it, you just gotta pay the price. In principle, used guns are no different than used cars, used tools and used women; they are not always what they seem to be.