I have three games, on Steam. Shadow of Mordor, Risen 3 and Lords of the Fallen. I have downloaded and played Risen 3 and Lords of the Fallen. I have only downloaded Shadow of Mordor.
Can I sell them to my friends so they can play them on their computer? I am unsure if they can, considering I have downloaded them on to my computer, already. Is it possible to transfer the files onto their computer so they can play them?
No, this is not something that you can really do. If you had bought the physical boxed copy, you might be able to sell that (although sometimes having CD-keys registered to an account makes ownership of the physical disks meaningless).
You are not generally able to transfer ownership of games purchased through digital distribution services like steam. Once you have effectively "claimed" the game by downloading it, the game is no longer transferable.
One thing you can do with Steam is register your friends as 'family', and then use Steam Family Sharing to share your games with them. There are some caveats, however. Your friends would only be able to play games from your library when your library is otherwise not in use. If you are playing a game, they can not play any games from your library.
You cannot transfer the games, but under some conditions you can ask for a refund.
Then what you do with the refunded money is up to you.
As Lohoris said, if you haven't yet played the game, or only played the game for a short while, and have bought the game on steam directly, you can ask for a refund. There are rules stating the refund will normally be granted if you have less than 2 hours of play time and you ask them less than 14 days after purchase, but it doesn't hurt to ask them for a refund even if the conditions are not satisfied - they might still say yes, especially if you never played the game and the price hasn't dropped significantly.
In practice you can also use a floating account - give the account data to your friends so they can log in with your account. But if you do use a floating account, things get inconvenient very soon, since a single account cannot be online and play games on different computers at the same time. So irrespective of the legality, this is very inconvenient and should only really be considered if you have an account that would otherwise be abandoned.
The third option is that you can buy most steam games as gifts. This would have been an option in the case of Shadow of Mordor, although it's too late now. If you buy a game as a gift, it needs to be activated before it can be installed. As long as the gift hasn't been activated you can pass it on to a friend, who can then pass it on to someone else, who can again pass it on to someone else - but the gift can only be activated once, only the one who activated it can play it, and it can't be passed on again after activating it.
