Stick only to the facts of your experience with this Craig's list guy and you should be okay. Avoid words like fraud, cheat, thief, liar, crook, etc. - any word that implies he always conducts business poorly. You don't know that, so don't say it. Defamation cases are based on damage to one's reputation - both personal reputation and business reputation - and you want to avoid that.
Comments and criticism based on one's own experience are legally protected free speech. Opinion is free speech. But too often opinions based on bad experiences with people wind up being defamatory, wind up wrongly implying a fact, and this can get you in trouble.
Truth and opinion can protect you in a defamation suit, but they will not protect you from having one brought against you.