It really infuriates me when sellers insist on setting up a device before sale. The instructions for selling or giving away a used Mac are easily found and they all include things like signing out of iMessages, iTunes, iCloud, ect. as well as deauthorising the device from their iTunes account. This ensures there are no personal settings or preferences left behind.
Then they just follow the first set of instructions I gave you, boot into recovery, reinstall the original macOS via WiFi the quit and shut down. Meaning what you should see when you start up would be the Welcome Screen with Hola, Bien Venue, etc where you pick your language, location and so on.
Just FYI, here are the instructions;
When preparing to sell or give away your Mac, you may want to erase some or all files from its hard drive. But first determine whether you should back up your computer and disable certain features and services. These steps show the recommended sequence of actions:
1.
Deauthorize your computer from iTunes, if applicable.
2. Back up your data using
Time Machine or other
backup methods for OS X Mavericks or
Mountain Lion.
3. If you use Find My Mac or other iCloud features on this Mac, complete these steps:
1.
Back up your iCloud content.
2. Choose Apple Menu () > System Preferences, then click iCloud.
3. Deselect the Find My Mac checkbox.
4.
Sign out of iCloud. When you sign out of iCloud, you're asked whether you want to remove iCloud data from your Mac. Removing iCloud data in this way makes sure that the data isn't also removed from any of your other devices that are using the same iCloud account.
4. If you're using OS X Mountain Lion v10.8 or later, sign out of iMessage:
1. In the Messages app, choose Preferences > Accounts.
2. Select your iMessage account, then click Sign Out.
5. Reformat your hard drive and reinstall OS X. Follow the appropriate instructions:
·
OS X Mavericks
·
OS X Mountain Lion
·
OS X Lion
After you reformat your hard drive and reinstall OS X, the computer restarts to a Welcome screen and asks you to choose a country or region. If you want to leave the Mac in an out-of-box state, don't continue with the setup of your system. Instead, press Command-Q to shut down the Mac. When the new owner turns on the Mac, the Setup Assistant will guide them through the setup process.
These are a little old but the principals remain the same.
Anyway so much for my little rant. Hopefully your instal of High Sierra goes smoothly and iMessage works for you. You will need to go to Contacts and fill in your personal data, phone number, email address etc as well and this should match the data in your Apple Account.
Let us know how it goes.
