Indeed, the threshold seems unclear, and increasingly it appears lower and lower. The last indictment of the Michigan LocalBitcoins trader saw him charged for < $150,000 in trading volume (rather, $56,000 that they definitively can link to BTC sales). That's a far cry from earlier indictments where people were trading for millions of dollars in value.
It really feels shitty that the authorities are throwing people in prison for this when there exists no clear guidance for what constitutes a "Money Service Business" vis-a-vis cryptocurrency trading. I think it's intentional on the part of the feds, as a general deterrent against P2P trading, because it's so hard for them to monitor on a large scale.