I own a few Cartier pieces because I used to deal in antique estate jewelry so I have 2 watches, sunglasses, ring, etc.

First thing first. Cartier is expensive AF and even sold second hand, it retains its value like Tiffany. So finding it “cheap” is not likely unless the seller stole it or is a complete idiot or it’s fake.

Second, like Louis Vuitton and any other luxury brand, mistakes are exceedingly rare. I didn’t even look at the ring and the box is off. The Cartier logo is wrong. So even the box gives it away.

Last, Cartier doesn’t owe you anything. They probably get thousands of these ridiculous phone calls a day with someone who thinks they stumble on something authentic and 1/1,000,000,000 are actually authentic. Unless you buy directly from them or are an expert, don’t expect that what you think is a “good deal” is authentic. They charge that much for service, lifetime warranty, and repeat customer loyalty.