Using a watermark preview does necessarily means that the track is unlicensed. It is not unusual for some editor to work with the preview before validation from the client and then forget to swap for the non-watermarked version. It’s worth finding out which case it is before insulting a potentially legitimate buyer.
Best course of action in such cases is to report to Envato support and to the author when they are known. They will investigate and take necessary actions when needed/possible.