We did it on a piece of paper held in the photo. Sure someone could still Photoshop, but it would help detour from pic stealing and reposting on here and FB
Yes I'm seeing that one Facebook and various place with a piece of paper. Try to place it so if you cropped the picture, it would crop your part too.
Some I see with their name and phone number. Like was said before, that's probably not great idea. Even your screen name over the picture would mean the person selling it would have to have the same screen name for it to make sense.
Anything with watermark is a deterrent. There's millions of parts on internet. So if you put watermark, a crook will just move on to the next picture.
I could just change something like this to autoxcuda 3-4 times for a for sale ad picture: