Look at it the other way. If you have a diamond in good condition, can you sell it anywhere for "market price" the same way you would gold or silver? No, because the rock is not rare, and there's basically no way to verify its origin outside of the store.

Look up Craigslist or Facebook Marketplace in your area and you'll find plenty of cheap listings for diamond jewelry. Will you really take a chance on any of them though?

Large, good quality diamonds are rare, smaller ones not so much (certain colors aside).

Not a few bucks, but for 10 percent of the valuation/retail price:

https://www.konkurser.dk/search/?s=diamant

It is in Danish, but it is bankruptcy auctions, the high price is the valuation (typically the retail price before bankruptcy), the low price is the final highest bid, all in DKK.

These are mostly very small and not the colors you'd want, are there other places? Otherwise don't really consider this as proof you can source diamonds cheap (would love to know if so where).

Edit: searched and found about 25 places, none of them have much inventory, most are lower quality (or incredibly expensive), many are in-person only with no real pictures you can see, closest I can find is https://www.catawiki.com but still for the type/size I've been looking for there's really only two options and both are lower grade.