Selling my legal game. What to do about the activation part?
Solution 1:
The FSX EULA makes no mention of it being none-transferable. I've "activated" the game myself on numerous computers, and it's never come back saying "You need to de activate on another device". Just uninstall the game, as you would when transfering it to another device; and that should be all that is required.
It does connect with MS when activating the key, but it doesn't do any such thing when uninstalling (just checked). So... there shouldn't be any issues. Your buyer should be able to install it just fine with the key.