1. Back It Up
Before anything else, you should ALWAYS back up your data on the old Mac to an external hard drive. To do so, start by clicking the Time Machine icon in the menu bar and choose Back Up Now. This keeps your data secure and limits the possiblility of any data that could have been corrupted in the transferring of data.
2. Deauthorize iTunes and Other Apps
It’s not too common for apps to have licensing schemes that are tied to your Mac hardware. If you have any such as Adobe Creative Cloud or Microsoft Office. If you have any be sure to deauthorize those.
In addition, iTunes is the one app that most people will need to deauthorize. Since iTunes allows up to 5 computers to be associated with your Apple ID.
To deauthorize, open iTunes and choose Account > Authorizations > Deauthorize This Computer and enter your Apple ID credentials when prompted.
This is another important step, since disconnecting your iCloud account will prevent your Mac from synchronizing your iCloud data between the two Mac computers.
To do this, open System Preferences > iCloud, and click the Sign Out button.
To unpair, choose the Apple Menu > System Preferences, then select Bluetooth. Move the cursor on the device you want to unpair, then click the remove (x) button next to the devices name.
*Steps 4 and 5 are optional if you are planning on erasing your Mac.
To start, select the Apple icon at the top left of the screen and click Restart. While the Mac is restarting, hold down Command-R to open macOS Utilities.
Now your Mac is ready for its new owner and can be reinstalled with a new macOS!