Prefer "S" or "E" models: Slim (2010+) and Super Slim (2013) run cooler, quieter, and have built-in Wi-Fi.
Compare total cost: Item + shipping + tax vs. local used game store with warranty.
If you accept the risk and do due diligence, eBay can yield a great deal. For peace of mind, a local shop with a 30–90 day warranty is often worth a small premium.