Does Applecare work on Third-party bought Macbooks?

I am going to buy a Macbook Pro in the near future, but I was hoping to buy it from eBay or somewhere cheaper.

Will I be able to purchase a used Macbook (probably 2011 model) from eBay and then purchase additional Applecare for it later on (if or if not Applecare is already included)? Or will I have to purchase a Macbook Pro from Apple directly and buy Applecare with it at the same time?