I don't think that YOU are doing anything wrong.... but they sure are. In my opinion either they would 1) most likely have NOT seen the deal through/paid even if you had just accepted their offer and not gone the auction route or 2) just another domain investor that "thought" they were making an offer that would allow for some ROI if they got it but the auction and no other bids gave them cold feet (doesn't remove their responsibility to see their offer through) or 3) a LONG shot but they could theoretically only be doing it HOPING that you send to auction, someone else wins, and then they can contact the winner with their OWN similar domain for sale.... dunno, a long shot but seems like a way to find possible buyers of similar domains and a built in "appraisal" too. Of course it sucks either way to have sales not go through but at least you also had the chance to run an auction at a reserve you were ok with and , despite the poor result, had exposure for your domains. I myself have only had that happen once, and sedo gave me the persons details. I did some research and found out that the guy had in that time frame lost his job and moved etc, so fits him not paying up.... but in my situation I wound up selling the domain for more about a year later anyways so all worked out in the end.
Hope you have better luck next time!