An amateur tattooist or "professional" of bad reputation and low-quality (and low priced) work. Usually an amateur or "a friend who works out of their house", scar merchants, or scratchers are unschooled, unsanitary and generally a big mistake. Going to one is literally buying a scar.
Dude, fuck that guy! Billy Bob is a scar merchant. Go to Tattoo City on Lombard for good work.