The small detail most likely to make a vintage watch worth tons of money

The vintage watch market is on fire right now. Watches are breaking records left and right.

No one knows that better than Aurel Bacs, head of watch auction expertise firm Bacs & Russo which has partnered with the British auction house Phillips. Phillips and the firm have sold eight out the last 13 watches sold above $1 million this past spring season, so it's safe to say Bacs knows what he's talking about when it comes to what is valuable in a vintage watch.

"Unrestored, original, in good condition," Bacs said, noting that the combination of all three in a rare watch is the thing that will increase a vintage watch's value the most.

And if you find a vintage watch in a relative's old things, the worst thing you can do is try to restore it in any way before getting it appraised. If there are visible signs of wear, don't worry about it. Even something small like polishing stainless steel can lower the value of a watch.

"Today people prefer unrestored with clear signs of wear than over-restored looking like new," Bacs said. "Even with flaws, even with stains, even with dirt, originality goes first."

Bacs compares vintage watches to luxury cars, explaining that vintage cars — even examples with rust or that don't run — typically go for a lot more than restored models at auction.

"Before you bring the watch to an auction house, don't have it restored at considerable expense potentially irrevocably, irreversibly removing patina, removing the original finish, maybe exchanging parts and losing value."

A specialist at an auction house like Phillips will be able to appraise a watch, without cost to you, and tell you if that slightly worn Rolex is actually worth serious cash, even if it's a little scratched or rusty.

"Go first to the specialist," Bacs said.

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