Any amateur tattoo artist that distributes and sells tattoos either without a proper liscence, or without credible experience. A tattoo from a scar vendor will typically become infected or it may not be what one originally wanted.
Friend: Dude, what's all that shit on your arm? It looks like someone took a belt-sander to your skin.
You: The fuckin' scar vendor on 3rd street messed my tattoo up.