Do I need to do a full re-install of MacOS before sending for trade-in or is erasing the HD enough?

Nope. Erasing it and trading it in without installing an operating system is perfectly ok. Just make sure there is no Firmware Password, that you signed out of iCloud, and basically complied with all the applicable instructions in What to do before you sell, give away, or trade in your Mac.

You are asked to erase the drive and reset it to original factory state. See What to do before you sell, give away, or trade in your Mac and Factory reset of your Mac - Apple Support. The latter can be done as follows:

Internet/Network Recovery of El Capitan or Later on a Clean Disk

     If possible, back up your files before proceeding.

You will need a fast, working Internet connection, preferably, Ethernet. If you must use Wi-Fi, you will be asked to select your network and enter the network password. This procedure will install the original version of macOS that came pre-installed from the factory.

1. Restart the computer. Immediately, at or before the chime, hold down the (Command-Option-Shift-R) keys until a globe appears.
2. The Utility Menu will appear in from 5-20 minutes. Be patient.
3. Select Disk Utility and click on the Continue button.
4. When Disk Utility loads select the target drive (out-dented entry w/type and size) from the side list.
5. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to Mac OS Extended, (Journaled.)
8. Provide a volume name, usually "Macintosh HD."
9. Click on the Apply button, then click on the Done button when it activates.
10. Quit Disk Utility and return to the Utility Menu.
11. Select Install OS X and click on the Continue button.

See this from Apple https://support.apple.com/en-ca/HT201065