Buying technological items second hand can be tricky business. There are three primary means of purchasing second hand computers. They are through a computer store that specializes in refurbished computers, from the manufacturer and from a third party such as eBay or Craigslist. Probably the most reliable ones are from a used tech store or from the manufacturer. Unfortunately they aren't the cheapest. If you decide to go with a third party dealer, make sure the site provides a user review system and you should inquire about what changes or updates they did to the internal components.
Probably the most important thing you should do when buying a second hand computer is bone up on some tech facts. You should be able to ask about the internal components, memory space, processor, running memory and other important facts and be able to understand their answers. A refurbished computer can save you a lot of money. But if some of the components are not updated and not compatible with contemporary programs you might be out of luck. Only buy from a reputable dealership, or through a third party market with a customer rating system.