This is a problem (for me at least) at any marketplace. The best thing these marketplaces can do is take a down payment for each offer which would be lost if the buyer walks OR at the very least, do some sort of email and telephone verification for offers. Yes, that would slow things down but trust me - it would weed out people who make crazy offers because perhaps they're just angry that you have the domain they want and they dont have it. I've had that happen to me quite often, getting cursed at by potential buyers because I seem to have purchased a domain 19 years ago when they were probably still wetting their diapers. But I digress.... point is, some sort of verification could help squelch these fake offers. IMO.