It’s important that if a device is being sold second-hand, the Apple ID must be disassociated from it. If you’re selling a device, you can’t just wipe out the drive and reinstall the operating system, you also need to disable Find My Mac and Activation Lock. If you’re buying a second-hand device, confirm with the seller that Activation Lock has been disabled and the product has been disassociated with the owner’s Apple ID. Also, confirm that you can contact the seller for their Apple ID info if Activation Lock kicks in and that you can return a device you can’t get access to.