The best way to value old shotgun shells is to contact a vintage ammunition dealer. There are gun brokers who specialize in antique weapons and ammunition.
Gun brokers who frequently participate in sales of old shotguns and old shotgun shells are able to provide the most accurate estimate of the current market value of an old shotgun shell. Old shotgun shells are far more valuable if they have never been shot, and those that are enclosed in their original packaging have the highest value.
There are a number of gun brokers who have online stores. Because of restrictions on sending ammunition through the mail, the best way to get an accurate estimate is to visit a local gun broker who specializes in antique ammunition. The old shotgun shells can then be priced and sold either by the original owner or by the gun broker. Since the gun broker is likely to have an established client base, the broker may already have contacts who are interested in purchasing the old shotgun shells.
Each state has different restrictions as to who is allowed to buy and sell ammunition, including old shotgun shells. Check applicable state and federal laws before attempting to sell old shotgun shells.