There have been a lot of people selling Mac computers on eBay and Craigslist that appear to be working fine. But as soon as you buy it, they will lock it through iCloud. Once your Mac is locked, you will try and contact the seller (If you still have their contact info) but will not answer or reply. So then you will take it to Apple for them to unlock it just to find out it is reported stolen and have Apple take your Mac so that they can give it back to the seller that sold it to you. Now that you just lost all your hard earned money, the seller will go and sell the Mac again to someone else and do the same scam.