I've heard it may prevent a sale, but I've also been told to put in the detail section that the watermark doesn't print and that usually works. I've seen a lot of photos sell with the watermark. Just look at the sold section and sometimes a lot of them have the watermark. I've also seen several images stolen off of FAA and used on websites (some have the watermark & some don't). I think it would be better if they had the watermark. I don't want it on all of my photos, but I'd at least like to have it on some of them. Especially the Alaska one I uploaded recently.