Hello MorayFalconer,
Being able to complete the setup and activation of your Mac is important. Based on the description you're provided, it sounds as though Activation Lock is enabled. To confirm, does the Apple ID hint not match the Apple ID you used on the Mac? If not, and it's the Apple ID is the previous owner, they would have to follow the steps listed below to remove the device from being associated to their Apple ID. You can also find these steps outlined in the following article. If you communicated with the seller via Facebook Messenger, you may be able to locate the conversation in your conversation history or the archive folder in Messenger. Those steps can be found in the second article linked below the following steps.
"Remove Activation Lock on the web if a device is offline
- Go to www.iCloud.com/find.
- Sign in with your Apple ID and password.
- At the top, click All Devices.
- Select the device that you want to remove from iCloud.
- Click Remove from Account."
How to remove Activation Lock
Archive chats on Messenger | Messenger Help Center
If the Apple ID resembles your Apple ID username, you can log in to the Apple ID to move past this screen. If you're prompted that the password is incorrect, and you're confident the hint that's shown is your account, you can reset the Apple ID password using the steps outlined in the following article.
If you forgot your Apple ID password
Have a great day!
