I've been surprised by how much people lie during classroom negotiations. To me, the downside of having people not trust you going forward seems like it should far outweigh the upside of winning an inconsequential classroom game. But, that hasn't stopped people.
One classmate has taken an aggressive approach to trying to get people to stop lying. During a recap of the how a recent negotiation went, this student directly called out the liar and announced that if anyone else lied during their negotiations they would not make a deal (even if it hurts both of them) and the fact that they lied would be shared publicly beyond the classroom.
In game theory, this is called the grim trigger strategy. This means that a player agrees to be cooperative, but will punish others as soon as they fail to reciprocate.