Per some online research from -SW Kolterman M.A., History Reno, Nevada USA
www.EdwardGibbonStudies.com
The biblio details are found in Jane Norton's Bibliography of the Works of Edward Gibbon, pp. 95, 103-05.
Yours is all or part of a 12-volume octavo (a standard 18th-century printing sheet cut in 8 pieces) reprint, referred to by some as the 4th edition. Vols. 1 and 7-12 are dated 1791, vols. 2-6 are dated 1792. The 'this day is published' advertisement was dated 3 Dec 1791. There is no indication of how many copies were printed, but Norton writes, "This is the first complete octavo edition" which contains for the first time the misprint "the solid fabric of human greatness", instead of "Roman greatness." I'm not sure if that increases its market value.
All books have all pages and covers and maps intact with the exception of one cover. It is not attached but we do have it. Other than that, we don't know much about these books. We recently started remodeling our basement and were getting rid of a lot of things but we kept these in hopes of selling them as they don't have any sentimental value to us. We want to find out the potential value before listing them for sale Any information you could provide would help us tremendously.
Thank you for your time!