My thoughts are that watermarks will make spotting scams harder.
Right now I can just reverse Google search an image and see if it has been posted on MM or IG/FB/Pinterest… Ect.
If the MM image has a watermark, and the other sites don’t, it won’t show in the reverse image results and I can’t see if it has been posted on MM before.
If sellers want to watermark their listings there are a million ways of doing so extremely quickly and for free… Some smartphones even have a watermark feature in the camera settings.