Whisker lists a Henry Sheets Sr. who worked early (as early as the Revolution period) in Shepherdstown (now West) Virginia. His son Henry Jr. worked from about the 1790 and died in 1825. In 1850 there was another Henry Sheets working in Missouri - don't know of the relation - the Sheets (Sheetz) family of gunsmiths was huge. Lots of Virginians (including gunsmiths like the Coles and Sites) ended up in Missouri so probably was a relative of the Virginia Sheets, and probably got his start in Virginia.
Without seeing any more details, my guess is that it is one of the later two, maybe the one who died in 1825, but hard to tell. Sounds like a nice piece. I don't know much about price, but if it is in decent condition, with all original parts, and was originally flint, a signed Henry Sheets fullstock with a patchbox sounds like a good find.