Something semi-simple that I've seen elsewhere - just require at least one photo of the machine in question that includes some sort of identifier for the account posting the ad. ie - I'm selling a Comet, so one of the pics includes a sign on the glass with "Beatnik-Filmstar" written on it.
Adding a note with your login to a photo isn't any sort of chore for an honest seller - you should be including photos anyway. And at least a fake seller would have to go through the trouble of trying to photoshop something to get past it.