The seller has to disassociate the device with their account or you have to reset the PRAM. A simple wipe and OS reinstall will not be enough. If you are doing

The seller has to disassociate the device with their account or you have to reset the PRAM. A simple wipe and OS reinstall will not be enough. If you are doing an in-person sale you should insist that the seller login to the machine with you watching, go to the iCloud pref panel and show that Find My Mac is turned off and the system has no associated iCloud account. Since having the device still associated with their account would give you access to their pics et al this is probably a good idea for the seller as well.