If you're buying it from eBay - look at the seller comments by other people that have rated he or she. Make sure you get a return-merchandise agreement out of the seller. If you're buying it from someone who has the camera - ask if you can use it for a day - and run it through every test you can think of - and get in writing a bill of sale that clearly states the seller will reimburse you should the camera prove faulty in some way - you can get the same result if you get the seller to agree to a 1 day, post-dated check. There are online websites that will help you evaluate the "health" of the camera - you want to be especially sensitive to any problems with moving parts, such as lens coupling mounts. Also, you're going to want to make sure the sensor is perfectly positioned - and that fine lines at the focal point center, are also as clear at the extremities of the image capture area. If you've purchased a new printer (with a scanner) you do something similar in testing / calibrating the print head(s).