Traditionally I have alway purchased my Apple products directly from Apple so I was not worried about getting a faulty product. But I am now on the market for a iMac. I was originally going to buy a refurb from apple.com but for some reason there are none for sale. My second option was craigslist. So to get to the point what are some things I should look out for when buying used. It there any how to tell-tell sign you are buying a piece of junk?