I’m always a little reserved about recommending used subs. There are good deals out there, but with so many variables, it’s easy to get a lemon. I would check Facebook marketplace, or Craigslist, or even a local pawn shop before eBay just to avoid shipping.
Lots of good brands, but I’d recommend looking around, and checking here with some that you find and we can help weed them out.
Having said that, if you can raise your budget to 300 and change, the Dayton sub1200 is a great deal(used to be 150ea now 157 but commonly have B stock for less). Your room is pretty big so you’ll need to spend wisely, and I would strongly consider two. Their sub1500 is also a pretty good bargain but ironically the 12” performs about as well. FWIW dual subs tend to be more for smoothing out bass across multiple seats but also, in big rooms will help with output.