The Watauga Purchase was one of several land purchases in the area of the Watauga River made around the same time. But this one was first, and we know this because it’s literally on Page One of the deed book, which still exists in the Archives of two different states (one is a copy, of course).
Interestingly, while this territory is sometimes considered by historians to be a self-governing area, the residents actually considered themselves to be British subjects, even after the war began, and to a certain extent even for a short period after the Declaration of Independence was signed. That likely ended when the Cherokees (who were aligned with the British) attacked several settlements, although they were badly beaten in the attempt.