This isn't a legal opinion (IANAL etc) but my understanding is both the auction company and the buyers are potentially guilty of receiving stolen goods. There is a defence if you can reasonably say you bought the item "in good faith". Buyers could reasonably expect the auction house to have done due diligence in ensuring items aren't stolen. So if I was them I'd be asking the auction house some awkward questions. If the original owners come calling though, the latest buyers will have to give the bikes back with no recompense. They'd have to take it up with the auction house, probably under small claims.
