What about picking up some macbooks at the local university library? Should you be able to wipe those and use (sell) them as well?

The activation lock is an anti theft mechanism so if you could just wipe it that defeats the purpose.

There are plenty of ways to do this without completely bricking second-hand devices.

For example, allow the Macbook to be wiped and reused, but inform the previous owner and ask them if it has been stolen. Yes? Trigger lockdown. No? Release ownership to the new user.