It does not matter which license is used. What you pass to the buyer is the copyrights. He can then choose any license.
When you make the agreement, note that you are transferring copyrights over the engine(code) and not the game assets. Also note, that you retain license over the game in it's original form, to distribute it and showcase it for any purpose without infringing on the copyrights of the game's engine(code). You will keep the code secure to protect the interests of the buyer in this contract.
So to keep things short and sweet:
Make a contract that states the following:
- The buyer receives full copyrights on the game engine(code) and receives no rights over the game assets.
- The seller retains the license to distribute the game in it's original form without modifying and/or reselling the engine(code).
This is no replacement for legal advice. In this case where you choose to avoid professional legal advice it may suffice to draft a contract like this yourself. That is your responsibility.