Sad. I haven't read the auction verbiage and am taking you at your word, so I wonder what recourse the buyer would have should they find out later on that it was a misrepresentation.
I haven't read the other thread either, so I don't know what the evidence is for that not being one of the original props. But looking at the auction, the serial number of the weapon is still readable, and Bapty is claiming that said serial matches their records for what was rented to Lucasfilm in 1976. On the strength of that, then, Rock Island hasn't misrepresented anything, so the buyer would have no recourse from them.
If it truly is a misrepresentation, the buyer would have to establish, in court, that:
a) Bapty's claim that this weapon was rented by LucasFilm is incorrect, and
b) that Bapty's knew or should have known that fact before they listed the weapon, or
c) that whoever consigned the weapon to the auction house knew that this was untrue.
Establishing those two things would give rise to a claim for negligence. If the buyer can also establish that Bapty's deliberately misrepresented the truth, that would give rise to a claim for fraud. Either of those claims would probably allow the buyer to get their money back, either from Bapty's or from the consignor, and might allow them to recover the costs of their suit. Of course, if they go after Bapty's they'd probably have to sue in the States and then sue again for Enforcement of Foreign Judgement in the England & Wales. (The United Kingdom is not a single jurisdiction for civil suits.)
That, of course, is assuming that the buyer and the consigner can't come to a deal without involving lawsuits.