But used coke is also worthless.

You’re confusing the wholesale price with the retail price. It’s not that used diamonds are inherently worth less, it’s the overhead of running a shop to sell them being a real cost. Walk by a jewelry store and you see both expensive inventory and people standing around waiting to sell to somebody, that’s a huge expense.

PS: Not to be confused with the third category of “wholesale” diamonds that’s simply retail with lower overhead.