I did a "trade" once, just to see what would happen. While I wasn't surprised he didn't send one back (and, quite frankly, wasn't really expecting him to hold up his end), it was his follow-up comments that had me jaw-drop at his brazenness.
First, he tells me he wanted a Champion, not the mystery skin I did send. I respond that he should have specified that from the start. He tries to finagle a second gift in the form of a champion. When I refuse, he has the balls to ask how much RP I have left. At that point, I'm just shaking my head in disbelief, tell him that's not info I ever share.
Then he gives a sadface, assumes I have no RP, while I'm staring the RP I'm holding back so I can buy DJ Sona during the Week 1 sale.
Again, I did it out of curiosity, not an expectation of a reciprocal gift.