Sorry, I read this post earlier and said I must respond but then the holidays happened. But I ran across mention of the Antiquorum issue again and it reminded me.
First, insofar as Timezone is concerned don't feel bad dropping off that list. The direction of Timezone had been compromised long ago into a place inhospitable to any sort of criticism of brands or practices within the watch industry. The place is also overrun with users that struck me as slightly bonkers fanboys for some of the brands and that was annoying enough. It is unfortunate because Timezone is also a depository for quite a few really useful historical postings.
On the stolen property I wanted to mention that the purported watch owner put Antiquorum in a really bad place legally. Before everyone piles on Antiquorum as a business calling them dishonest or worse, they need to carefully consider the situation from all perspectives. (Never thought I'd be an apologist for Antiquorum, but fair is fair.)
First Antiquorum is an international business and ownership laws are very complicated already -- even more so internationally. While the person presenting himself as the original and rightful owner does in all likelihood have a valid claim, it is difficult for a business to legally verify the situation. The best course of action for Antiquorum was indeed to back entirely out of the situation. It was a no win situation for the business and fraught with liability issues from both sides.
There is no telling how many hands the watch has passed trough and the consigner might have had an entirely legal Bill of Sale from a reputable business. It is also difficult to verify how the watch originally entered the used marketplace. I accept the OP's statement that the watch was stolen. But how can a third party legally verify the bona fides of that claim?
I am in no way trying to impugn the character of the OP, but as a disinterested third party consider the possibilities. The watch could have been surreptitiously sold in any number of ways for any number of reasons. Proceeds from a divorce, pawned for cash, an insurance scam, simply lost, etc. etc. (Again, not impugning the character of OP in any way!) It is a question of what can legally be proven. A business cannot evaluate these competing claims.
Here is what the OP should have done from the beginning. First contact the auction house and ascertain where the watch was located for pre-auction viewing. Next, contact the police department where the original theft report was filed and report that you have located some of your stolen property. Give them the current location of the item as reported by the auction house.
If the watch was insured, the next step and biggest help would have been to then report the situation to the insurer. Insurers have well established procedures to recover stolen property.
After those steps, the next step would depend upon how much you value the item. If it is very precious to the owner, the next logical step would have been to contact a lawyer. A lawyer could have obtained a court order impounding the goods as stolen property to be further litigated by the claimed rightful owner and the consigner. If the item is just a more run-of-the-mill asset, you might forgo a lawyer and simply directly contact the police department where the item was located.
Unless the item is located in the same jurisdiction from where it was stolen, the next steps are difficult and you might need a lawyer in any case just to maneuver through the situation. This is why I said it would be helpful if an insurer was involved.
These steps are what the OP should have undertaken. By contacting Antiquorum directly as a first step, you've allowed them to do exactly what they should have done as a disinterested third party if given the opportunity — namely withdrawn from the entire situation.
What is done now is done. The only ethical expectation that one can have for Antiquorum at this point is for them to willingly assist the police during further inquiries.
My opinion at least.