You should read this article:
http://www.wikihow.com/Copyright-SoftwareIt does not matter which license is used. What you pass to the buyer is the copyrights. He can then choose any license.
IfWhen you make the agreement, note that you are a US citizen gotransferring 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 site:contract.
http://www.copyright.gov/ So to keep things short and sweet:
After that askMake a lawyercontract 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 draft aavoid professional legal agreement for passing the ownership overadvice it may suffice to the buyerdraft a contract like this yourself. If itThat is not worth the investment in a lawyer I suppose you should ask the other party what would be sufficient guaranty in his opinionyour responsibility.