Not a designer but think best thing for a template would be to showcase and highlight the design like our templates and think it would make sense to transfer the site once they pay and deciding if they will customize it themselves or hire you to do it.
If they want you to customize it, you could leave yourself as a contributor of the site.
https://support.wix.com/en/article/transferring-a-copy-of-your-site-to-another-wix-account