I am planning on purchasing an unopened mid-2012 13" tomorrow from someone who contacted me through craigslist. Is there anything I should look for or be aware of before I make the transaction. Also, I asked for a serial number, but the person said that since it has not been opened or registered he cannot give me the sn. After some reading it appears that since it is unopened this is actually a good sign. I plan to open the box before leaving the public meeting place. thanks in advance! can't wait to have a good mac again!