New Guy , the firearm you are discribing is called a "CAPE GUN", they are usually rifle on the right barrel, and the shot barrel on the left. Most are 16ga, but there again most are chambered for the 2 1/2" shell, and are mostly very full choke. The 9mm on the rifle barrel will have as much to do with it's value more than anything else. If it is a 9x57R it will be easy to handload rounds for it, however, if it is a 9.3X72R is not a very popular cartridge, and will negatively effect the worth. If it happens to be a 9.3X74R then the price, they are asking, is cheap, if the gun is in excelent shape. The shot tube can be opened up to 2 3/4" , and the choke opened up to Modified at not very high cost, but must be done by someone who understands doubles, especially double rifles. Most all smiths do good work on things like K-Mart Mod 700 Rem, but you have to be very careful who you let touch your doubles. A blunder on any double can be a disaster, and completely destroy it

I have a couple of these cape guns, and they are very handy hunting guns. Is the one you are looking at a hammer gun, or hammerless?