Kim, Welcome to the group.
You may already have the marriage of your ancestors Thomas and Sarah:
Archbishop of Canterbury Marriage Allegations:
4 Jul 1761. Thos WELLER of Tunstall, bach[elor], and Sarah OLIVE of s[ame parish] sp[inster], at Tunstall.
There is no baptism record of a Thomas WELLER in Tunstall in the relevant timeframe, but a possible candidate is a Thomas, son of Thomas and Elizabeth WALLER, bapt 15 Jan 1731/32 in neighbouring Milton-next-Sittingbourne [Source IGI] There are several WELLERs, children of Thomas and Sarah in the years following, so the 1731/32 Thomas may be son of the same parents.
There is a marriage at Milton after banns on 24 Oct 1725 of Thomas WALLER and Elizabeth HEARNDON. Also a burial there on 22 Oct 1745 of Elizabeth, wife of Thomas WELLER. There is no record of the burial of an Elizabeth WELLER in Tunstall in the relevant period, so she may be the one who married in 1725. There was a burial in Tunstall of a Thomas WELLER 30 Jul 1806. [BTs.]
Corroborating evidence is necessary before it can be accepted that these refer to the same Thomas and Elizabeth who are your ancestors.
Richard