Don't be a jerk to a player when they do not want to trade you their champ. Being a jerk is completely not necessary because all it does is make sure people are interested in helping other teammates over you and causes discord among your ranks.
I just played with this super rude player who started bitching at me because I wouldn't trade him nami. I told him in pre-game chat that I didn't like the champion he had and then said that I had better be mastery 7 with the champ. Then game started and he saw I wasn't mastery 7 and he decided he needed to be a prick even more because I didn't feel like wasting blue essence and tokens on upgrading my mastery (since we veterans were horribly shafted with blue essence from runes).
I told him to focus on the game and he called me rude. I just brushed it off and focused on the game myself and not his awful attitude.
We win and I end up doing more damage than him and having a higher kill participation than he did. So not only did he perform subpar, he was negative to his teammate for nothing.
Always remember, don't be a dick to your teammates who don't want to trade you the champ they have in ARAM. It's better to start off the game on a good foot with your teammates rather than creating hostility before the game starts.