Usually "rebuilt" batteries are cores or used batteries that the shop puts on a charger and load tests. If they pass they clean them up and sell them as rebuilt. In Texas you have to call them "used". It's a crap shoot though. A lot of batteries get replaced that are perfectly good but you also get marginal batteries that can pass the load test right off the charger but will not last in a car.
Agreed though, I'd never touch a used or rebuilt battery. Always buy new. Also be aware that 90% of the batteries you see on the shelf are made by Johnson controls so it doesn't matter if you buy an Autozone, Napa, OReillys, Advanced, or Walmart battery. They are all identical.