I don't know the process but it happened to me, eBay purchase and when I checked after a week of having the computer, suddenly no Apple care, The owner had posted the Apple care receipt on the listing, someone, not even the original owner had seen that info online and used it to fraudulently get a refund. Fortunately I had an honest seller who contacted Apple and I was given a refund for the applecare.
Perhaps there is a way to contact Apple and make sure the apple care is transferred in name to you? So that the original owner or anyone else can't do get a refund.
I'd like to think that 99% of people are honest and if you are careful you should be okay.