How is this different from icons on LJ or "flair" on Facebook? It's different because you're making money off it. I'm not going to rat anyone out, but... it's a bad idea. And I don't know about Etsy, but I know on CafePress that sellers have to agree in the terms of service not to sell anything they don't have the rights to, so the same goes for them, too.
And who knows--maybe they're too busy rushing out sequels on a five-dollar budget to go after you. But I know other movie studios have done it, and some of them are testier about it than others. If you want to roll those dice, be my guest. Just don't ask me to contribute.
ETA: Hm. A lawyer weighs in.