Is your work copywriten? Is your avatar your own work?
In this case it was fan art that I would not sell myself because of copyright laws, that somebody else has taken on themselves to scrub off my signature and watermarks, upscale (badly) from a low res upload and sell on their Amazon shop as an on-demand print without my permission. The law protects me as the maker of the fan art, the image I made is copyrighted to me, and CAPCOM remains the owner of the copyright of the character in the image, as per the Berne Convention/Creative Commons license.
If it was one of my own copyrighted works or characters then I would go to town on this guy. As it stands all I could currently do was to politely ask for him to remove it from his storefront, before taking it higher, since it looks bad on me or that I might be mistaken for endorsing the mass sale of copyrighted works. It looks like he has removed it now anyway.
The last dude who did this on eBay decided to wrangle with me that I didn't have the right to tell him to take down my work because it wasn't "mine" when the character belongs to CAPCOM (not like he had CAPCOM's permission to sell it either), but he's legally incorrect, and I got him to take it down in the end. Legally it's mine but it can't be sold, only shared, and I nor anybody else can profit from it in that way. In reality a lot of artists do profit from small-scale copyright breach, such as dojin publishers and print makers at cons, without getting legal permission from the copyright holders but dojins usually make next to nothing in profit after costs and small scale runs at cons are normally beneath the concern of large companies. Print-on-demand mass selling like this guy though would attract their attention.
Don't get me wrong, I don't mind or care if someone uses my work for their own amusement, wallpapers or whatever... there's even a few people who have used my work as avatars and sigs on this site probably without knowing they're mine and don't mind at all. Someone just starts selling it like this though and it's in my interest to get them to quit. I don't want anyone coming up to me online at some point and saying "hey, aren't you the dude who sells DMC prints?" because I don't.
The av isn't mine, but I know the artist. I've seen people trying to sell her work of copyrighted characters, too.
I just draw the line here: realistically anything I upload to the net is fair game for someone to use, change, deface, whatever. But as long as they aren't selling it, it doesn't matter. As soon as they start trying to mass produce for profit, we got problems.