Oh no this has been happening for more than 10 years. My Beetlejuice lineart has been on the web so long that people think its stock art and not even fanart. I see it on stuff everywhere, ebay and etsy etc. People copy them and post them on deviantart (at least that's not for profit). They've even been printed in magazines, along with the episode guide of the cartoon show I wrote ages ago. People think its all official and free to use. I just saw a lady selling a crochet pattern she made from one of my drawings on etsy, so OTHER people can take it and make stuff with it. -_- It just makes me sick inside. There's nothing I can do about it, since its been out there so long no one can trace where it came from.
edit: here's another one. So speaking of etsy I joined today to sell my stuff on there. My company name is "Sparky's Stitches" but that name is taken by someone who not only isn't using the account but doesn't look like they ever did. But there's nothing I can do about that.
So I guess my shop will be called "NeitherSparky". Bah.