U can look up the warranty expiration date
on apple.com/support by entering the serial number. That will show you the AppleCare expiration date and it will be valid.
I've purchased a couple of used Macs through CL and gotten service on one of them. Apple doesn't require a receipt unless you believe the online warranty expiration date is incorrect.
To protect yourself, I suggest you get a written and signed paid-in-full statement from the seller that states model & serial number.
I called Apple customer service once to inquire about this scenario and they said they had no way to track stolen merchandise and would honor the warranty attached to the serial number of the computer.