As I said in my message, I covered that with the "retailer premium", which I understand exists for a reason, which I describe. That reason does not cover the degree to which you can not resell the ring, which is absurdly large. Also observe that despite technology recently being able to make "forgeries" ever better, this inability to resell at anything like what you paid for it has been true for decades.
Maybe because the demand for rings is not for their commodity value, but for their signaling value? So buying a ring from Tiffany's is worth a lot more than buying one off craiglist? And they aren't scarce so jewelers don't need to snatch them up off the "open market", but they need to make high margins to cover the overhead of their fancy stores.
Are you looking for a cause to the "absurdly large" degree? Or just pointing it out as some mysterious conspiracy?
Of course you can't get full retail value when reselling a luxury good. That's as true of Tiffany rings as it is of Macbooks.
But there is a conspiracy to exploit people desperate enough to have to liquidate their jewelry. It's detailed in the article linked to by the OP. That conspiracy is what causes jewelry to have a resale value of maybe 20%. Compare this to 6-month old MacBooks Air (worth ~80% of their retail price) or a car you drive off the dealer's lot (worth ~80% of what you paid).
