Sherry rifles are usually high quality guns with significant art work in the stock carving and the patchbox and other brass work. Impossible to evaluate without a good set of photographs that covers the entire gun, both sides, from butt to muzzle. Usually "standard" half-length shots from butt to about 8" past lock plate area on both sides of rifle, plus a full length view of front side [all in focus], and detail [close-up] shots of all important features, will get it started. The only way to get a good appraisal is to work with a firm, or a person, who is knowledgeable about rifles from this area of PA, and has seen and handled Sherry rifles in the past. Large auction houses that specialize in selling early American rifles can help, such as Cowans in Cincinnati, Morphy's in PA, Rock Island Auction in IL, etc., all have experts in antique arms, and usually have access to prior sales of similar or comparable rifles and are willing to help...especially if they think you might be selling it some time down the road. The Kentucky Rifle Association, or KRA of which I am a member, is the premier collectors group for better Kentucky rifles, and has many members who are advanced collectors of PA longrifles, have bought and sold them, and would have good knowledge in this area. Contacting their web site and asking for help would get you hooked up with a knowledgeable collector. BUT, always remember that most collectors will probably be interested in the rifle themselves, and may be a little low in their estimates, "just in case" it might come their way. Hard not to be!
Most antique dealers do not have the specific knowledge to properly appraise a high quality Kentucky rifle; they work on generalities, often missing the mark on a really outstanding rifle. So you need to work with someone knowledgeable about better PA rifles, and familiar with Sherry's work and what his rifles have brought in the past, or having access to prior auction sales of similar rifles. We on this site are not supposed to discuss specific value in the postings, but if you post the 3 "standard views" of the rifle that I mentioned above, we can advise you of how it stacks up against other known Sherry rifles, so you will know if you have an average, or perhaps very fine, example of his work. Shelby Gallien