"This is interesting, but I guess I'm wondering how it really differs to buying a brand new car. The moment you drive it outside the car dealer you've lost money."
A: Nowhere near as much, and B: That's what I was referring to with the "vendor guarantee"... the reason why your car loses its value when you drive it off the lot is obviously not that the car changes meaningfully, but that in the somewhat scammy world of used cars you will not be able to establish to anyone that your car really is brand new anymore, so you take a hit based on the fact that you do not have a reputation that says "I am $CAR Dealership XYZ and I stake my name on the fact this really is a brand new $CAR straight from the factory".
(Incidentally, to some extent, note that when people suggest that the solution is to always buy used, they often aren't taking that into account properly in their zeal to make their point. You're paying less precisely because the odds of you being scammed just went up. If you are, for instance, someone very likely to be able to detect scams, by all means, go collect your "knowledge premium" from the market and buy used. But if you are not such a person, think twice. I don't mean "never do it"... I just mean, think twice. I have family that was taken for a ride by buying used when they "couldn't afford" new... well, it turns out they couldn't afford "scammy used" either....)
Yes, that is one solution to the conundrum, buy at actual market value. In the longterm that's an unstable solution... if rings lose their signaling value entirely (see other comments), then the eventual stable solution is not to buy them at all. However, in the meantime, what the eventual stable solution 50 years from now will be isn't actionable today, so by all means buy something that impresses your friends and pocket the difference. My wife and I went a different route and bought a nice ring with two Tanzenite stones in it for, IIRC, on the order of $250 1999-dollars, and I still think it looks pretty nice. Tanzenite is a bit soft but we've had no problems, and I think it's beautiful but the jewelry community seems to think it isn't worth much. We were happy to come to arrangements with them on that matter. This is another way to hack the process, since people aren't consciously thinking all the way through this it can suffice to present a token similar to a "diamond ring" but that did not require the useless investment.