You don't have to go through a jeweller to re-sell these days though, meaning that you can get a lot more for your jewellery. I see your point though. But at least you can get SOMETHING back, unlike many other things that people spend their money on, like holidays!
I've never understood why indulging oneself with frequent holidays, a fundamentally lazy and expensive thing when done to excess, is seen as socially acceptable while buying jewellery isn't. Jewellery is tangible and lasts forever. Same with cars that are much better than needed to get from A to B safely, expensive phones, expensive hobbies. Jewellery is the one thing that lasts and that you can make money on, if you know what you're doing:
Your post inspired me to look up the current selling price, new and second-hand, of a piece of jewellery of mine. There's a jeweller near me, been there since 17-something, and they have a lot of estate pieces. About 13 years ago, I bought a Tiffany yellow-gold Etoile ring from there. A plain yellow band studded with diamonds. The diamonds are ringed in platinum, unlike most scatter rings which just have the diamonds pressed into the metal. Anyway, I paid $635, and it was in stunning condition. I just checked recent ebay sales of this ring, and there are two - $1,900 and $2,450. The cheapest price I can see online is $1,450 from a jeweller, for an unpopular size and with wear showing.
From the same store in the same year, I got a second-hand Cartier Entrelaces ring, which looks like a metal knot, for $450. It sold for $1,300 new at the time. I'm sure I could get a good price for it if I sell it 30 years from now.
You can make money on jewellery if you buy at great prices and they are really good pieces, and if you hold onto them a long time. I bought most of mine well over a decade ago, so I'm lucky I guess, since gold has surged. Not that I'm selling any of it for quite some time.
I think that the Kate earrings will hold their value, especially given that she will be queen. They're famous and have the royal seal of approval, and such things matter when it comes to pricing. OP may not be able to get all her money back if she sells them in, say, ten years, but she'd be able to get a solid chunk. Might even make it all back or more if she sells them, say, 40 years from now. The good thing about this kind of jewellery is that if you look after it, it will look good as new 40 years from now.