MDM Activated after Reset 2018 Macbook Pro when I didn't have MDM before

If and only if there is truly a Mobile Digital Managements( MDM ) Service installed on this computer - it can not be removed except by the MDM Administrator

Regardless of Wiping and Reinstalling the computer - the Serial Number of his computer is Registered with the MDM and can be invoke at any time by them.

Use configuration profiles to standardize settings on Mac computers

Find your proof of purchase and contact AppleCare. Also, the message should indicate the company that owns the MDM, so have that available to.

Again, with a copy of your proof of ownership, send a cease and desist letter to the company.

The why it has appeared after this time - honestly no idea.

I tend to conceded to my more learned colleagues assessment.

This may not have been done purposely but it has been done.

As @Barney-15E has already suggested remedial action on your part, not much more to add except best of luck

I did purchase the laptop on eBay in 2018.

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Is there some way to remove the MDM lock?

The only way to do that is for Turner Broadcasting to de-provision that Mac.

I know that's not what you want to hear, but if that Mac was provisioned its serial number is in their system and only the MDM administrator can remove it. Erasing the Mac will not help.

You may have difficulty obtaining proof of purchase from the eBay seller that Turner Broadcasting would consider acceptable. Unfortunately only a tiny minority of eBay sellers correctly prepare used Macs for sale.

Perhaps Turner Broadcasting sold that Mac legitimately and simply forgot to remove it from provisioning. Or perhaps they entered the serial number in error as Barney-15E suggested. On the other hand perhaps it was stolen from them and they're going to demand you give it back. It's not a good situation.

It is managed by serial number. Someone probably fatfingered your serial number in their MDM account.