Yep, I agree with Mike...the "free", but somewhat time-consuming method, would be to right-click over your picture...open it with the paint program, and then type in, around, where-ever you want, so that others will "hopefully" not steal the photo...
That watermark feature here doesn't work...you can still copy the photos by doing control-c....but, all-in-all, I'm thinking it really depends on what you're selling too. For instance, if you're selling "wholesale" items like SMC or just some other average-joe item, others can easily get the factory photo and things like that....but if they're blaintenly ripping off photos with your blue lamp in the background (for instance), you'll just have to report them to ebay, their auction will get pulled, and if they do it enough...they'll get shut down.
However, you're just going to have to take a few extra steps if you're concerned about your photos that much...."nothing in life comes easy" ya know...lovely cliche isn't it...LOL.
So, if you use windows, open the paint program and start "typing" over your pictures.....have fun
Best regards,
Kathleen