Trade at your own risk (to those wanting to take him up on his offer). Mystery Gifting is intended for gifting, not trading, and Riot does not condone it (though it is not against the rules). If you are cheated out of a gift trade, they will not likely refund you.
If you must do trades, you should prefer to do it with someone you know personally. It's not impossible for someone that is just a friend through League to take your gift and unfriend you regardless of how long you've known them.
Giving gifts as gifts and not expecting a reward is the right way to do it. Having a good reputation with friends and giving them without expectation often leads to getting them back too. 