I currently own a used 13" Macbook Pro (2010 model) It was 5 months old with 3 charge cycles on the battery when I bought it. The original owner took excellent care of it as it never left his house hence it has no marks, scratches or scuffs.
Functionality is perfect.
I saved over $350 going this route.
Apple standard warranty is one year, I have 6 months left. Applecare can be added for an additional fee before the one year cut-off date.
Just do your homework, hopefully the seller will have good reputation/feedback in the online community, can share some photos, information and will spend sometime with you to demo it when meeting up. Excellent communication is important and goes a long way when making an awesome transaction.