Transfer Lion with my MacBook Pro sold to 3rd party I had a old MacBook Pro with Snow Leopard (OS X 10.6), then I bought the update to Lion (OS X 10.7).

Then I bought a new MacBook Pro that already comes with Lion and sold the old MacBook Pro. So, technically, I have two Lion licenses.

From the license contract on the App Store, I read that it (or Mountain Lion at least) is not transferable. I guess that means I can't transfer my license away from my Apple ID.

But I think it makes no sense I have two licenses and the person who bought my old MacBook Pro would have to buy a new (3rd) one to install it and maintain it. Is there any fairer solution? Would that be illegal?

Of course I could just install it using my account or whatever, but I already did that and now he wants to re-install it for whatever reason, and that's basically where all problems started.
The licenses you bought on the Mac App Store are non-transferable, as you already found out. The license for the preinstalled copy CAN be transfered, but only as part of a transfer with the original hardware it was installed on.

Therefor the only thing you can do when selling your old computer is delivering it with the original software it came with. Anything else would be a violation of the license agreement, and can be considered as software piracy.