If you suspect someone will be committing fraud, report that to a mod. My idea is to act like a honeypot. Rather than blasting that guy right away or call the deal off, report first to a mod. Keep doing the deal and make your lost small. Pretend like you have no defense against fraud. Give all evidence to the mod. Once the evidence is obvious, blow things up. Here, a mod knows who is right and who is wrong from the beginning. Quite often, after the fraud happen, it's the fraudster word against the victim and it's hard to verify which one is the asshole. Also fraudsters often have nothing to lose. We don't know their real names, their account is new and stuff. With cooperation from mod, the fraudster can give their name to some trusted person and we can destroy his whole life once the fraud is obvious.