Enter a frame serial number to see if it's been reported stolen on CrankMart or Bike Index. Always a good idea to check before buying a used bike — it only takes a second.
Reminder: a clean check isn't a guarantee. Serial numbers can be filed off, reports take time to appear, and many thefts are never reported. Pay through Buy now where you can, and ask the seller for a photo of the frame serial number and proof of purchase.