It turns out there is another ebay seller doing the same thing, and they're also trying to promote their business. Nothing wrong there.
Except, they took the picture of our license plate and put it on their web page. And not only took the picture, but actually hotlinked from our site. What that means is that they are directly using the picture hosted on our server, using our bandwith every time somebody pulls up their page in order to see our license plate.
Well, the unintended consequences of this can be quite painful, as you can see here. So for as long as they fail to notice that we've noticed the theft, all of their potential customers will get a marketing message quite different from the one they intended.
They will learn to be careful with those petards.
UPDATE: They pulled their entire page. Since it was the only marketing site they had on the web (we could not find a web page for them), they're left with nothing until they figure out how to get pictures hosted themselves.