I’ve bought a second hand Mac boo but it still has the ex owners Apple ID on it. What do I do

See Turn off Activation Lock https://support.apple.com/en-us/HT201441. All possibilities related to iPhones, iPads, and iPods acquired second hand are covered here except the case of inheritance.

Since If you acquired your device second hand, you usually will need to obtain the needed password from the prior owner or have the prior owner sign out of iCloud; erase the device; and remove it from their device list. If unsuccessful, in the rare case your device came activation locked from an authorized Apple seller, then contact Apple Support and present your original purchase receipt for your device with the device’s serial number on it. You’ll need a photo ID too. If you can do none of the above, then the device is useless to you.

You can use the suggested support options below if you have the needed receipt. Alternatively, if an Apple Store is open in your area, then you can go there to deal with your problem. In this case, you can make a genius-bar appointment (https://www.apple.com/retail/geniusbar/) if you want.

Get a call from Apple support to help solve your problem using this

link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362

Apple ID Support https://support.apple.com/apple-id

Apple Support Twitter  https://twitter.com/AppleSupport

Apple ID account security for country-specific andregion-specific help https://support.apple.com/en-us/HT204169

Get Support: We’re here to help https://getsupport.apple.com/. Go through the options to find the best support option.

Start with step 6.

Selling old Mac 

Perhaps this may help: https://support.apple.com/en-us/HT201365