Perhaps, but so what? At that scale there will be huge numbers of both false positives and false negatives in any content moderation system. If you dig into any popular online classified sales site you can find some illegal items.
Criminal activity is quite a different thing than censoring legal content which possibly violates corporate terms of service. If you have evidence of an actual crime then you should report that to law enforcement instead of expecting a private for-profit company to handle the incident.
And absent further hard evidence, I am frankly skeptical of your claim. Most people aren't experts on the nuances of firearms sales laws in various jurisdictions, so a post that appears to be soliciting a crime might be entirely legal (or vice versa). I don't really use Reddit, but I've been on Facebook for years and have never seen a post for an illegal firearms sale. Do you at least have some screen snapshots?
