Flintlock or percussion . . . or converted to percussion from flint?
Single barrel or double barrel . . octagon or round or octagon to round?
Wood that stock is made from . . . walnut, cherry, maple or what?
Actually a smoothbore or a rifle?
Type of furniture . . . i.e. brass or steel?
Fullstock or halfstock?
Original or repairs . . . cleaned or in original state?
Before shelling out $$$ on any antique weapon you need to educate yourself on these types of things . . . . I'm not being critical in that remark, I'm being realistic. It can save you from making some very serious mistakes and overpaying for something that is not worth a whole lot. From your description and what the seller says, it doesn't appear as if he is very knowledgeable about it either. What is believed to be an "antique" could end up being nothing more than a cobbled up gun made out of old parts that are "married" or a reproduction that has been "aged" and represented to be an "original antique".
Oftentimes, someone will find an old gun and try to make it look nice by "cleaning it up". In essence, they are ruining the value in removing the old patina and many times, removing markings by getting carried away with emery cloth. All of these things will affect the value.
If you can post some photos, it will make it easier to determine just what it is. No photos . . . it's anyone's guess.
If a pair of '51 Navies were good enough for Billy Hickok, then a single Navy on my right hip is good enough for me . . . besides . . . I'm probably only half as good as he was anyways. Hiram's Rangers Badge #63