An easy way to do this, for people who may not know how to apply a watermark to their photos, just take a 3X5" index card and write your user name and the day you're starting the auction. Include this with the item in your photos. This will also make buyers more comfortable when bidding on your item.
It's also important to note one thing, though: Any picture, watermarked or not, can be successfully edited to remove that mark. It's not a guarantee, but it is a very strong deterrent...
