The Ballon Bleu with a moon. Automatic, with its box and card.

Cartier is the house that made the wristwatch a shape rather than a circle: the Tank of 1917, the Tortue of 1912, the Tonneau of 1906, each a case with Roman numerals, blued sword hands and a cabochon in the crown. Cartier makes them in gold and in steel, in quartz and in mechanical, and on the pre-owned market the gold ones sit where a Rolex Datejust does in price and a step above it in how they read across a table.

The Ballon Bleu of 2007 is the round Cartier with the crown tucked into a guard, and the Moonphase of 2016 put a moon disc at six under a silvered guilloche dial with the in-house calibre 1904-LU MC driving it. At 37mm it is the mid size, the one that reads unisex. Rebag’s unit has its box and a warranty card without a date, which is more than most of the Cartiers on this page come with; the grade is Very Good, which at Rebag means light scratches and wear rather than dings.

Rebag gives the size as 37mm, the thickness as 13mm, the band width as 18mm, the wrist size as 6.5 inches, steel for case and band, a silver dial, an automatic movement with a moon phase complication, box and an undated warranty card, and Very Good with scratches and minor wear. It gives no reference; WSBB0020 and the 1904-LU MC are the catalogue match and the copy says to confirm. Water resistance is Cartier’s figure for the model, marked confirm.

Rebag authenticates in-house and is not a Cartier authorised dealer, and Amazon adds its 30-day return window. Ask for the reference and the numbers on the caseback, close-up photographs of the wear, and when it was last serviced. Nothing here was measured on the watch.

This unit is sold by Rebag through Amazon Luxury Stores, authenticated by the seller with Amazon’s 30-day return window on top; it is one piece, and when it sells the listing goes away, so the second button opens the live search for Ballon Bleus.

Steel Cartiers have resold close to what pre-owned buyers paid, sometimes more, sometimes less; that is a pattern, not a promise, and nothing here is investment advice.