Yes, if planning to sell - wipe the drive of all partitions and then install Sierra. This can be done via a Bootable Installer of Sierra.
Once hat is done - disconnect all external drives and shutdown. Attach to charger and connect only the USB Installer. Restart and Immediately hold the Option key. This will present the Boot Manager and choose the USB Drive ( Installer ). That will present option, choose Disk Utilities >> View >> View All attached devices. Now, choose one of the partitions at bottom and look for a small - ( Minus Sign ) >> click it. Do the same for each Partition until there is Only One Partition showing. Next step is choose the Upper Most Drive usually called Apple SSD and choose it. Click it and choose Erase >> give a name >> format as HFS Journaled and below that Scheme: GUID Partition Map >> Erase.
Back out of Disk Utilities and presented with options >> choose Install macOS. This will start Installing Sierra and may automatically restart several time until it finishes and Setup Assist will be presented.
Important: If giving / selling Stop at that point and go no further. Just usage the Power Button to shutdown the computer. The next person who turn on the computer will the experience like as Brand New Computer and Setup Assit will automatically be presented.