@CapitalG , I have some point to clarify:
1- Why would you pay for an eight years old macOS 10.8? What mac do you have?
2- macOS 10.8 Mountain Lion will be no longer supported very soon, as all new apple software run only with macOS 10.12 Sierra / macOS 10.13 High Sierra and latest macOS 10.14 Mojave. Consider also macOS 10.15 Catalina coming very soon.
3- If you want to reset you mac, you should run Internet recovery by restarting your mac into internet recovery mode. Shutdown your mac, Start up again and keep holding these keyboards together: OPTION+COMMAND+R , on some old apple mac models is: SHIFT+OPTION+COMMAND+R. Your mac will go onto Internet Recovery mode and it will pick the appropriate macOS automatically.
4- You say you have purchased that macOS 10.8 Mountain Lion. If you did so so you have done it by purchasing with your own apple id. That is enough to install it on your mac. On your setup procedures you will be asked to fill in your Apple ID with your password.
5- If your mac was locked by the previous owners, you have two options: 1- Contact the old owner and ask for unlocking your mac. 2- Contact apple Support and provide the POP "Proof of Purchasing" document. They might help you.
I hope you found your answer here. If so yes, then please confirm it so the Admin can close this thread and mark it as completed.