Do you mean the monolithic ceramic capacitors? I don't know of anyone that advertises buying them specifically, but you can get them toll refined. This being because not all of them contain precious metals, and unless you want to go and test every one, there's no way of knowing if they are worth anything. If they are older than 1992, then there is a much better chance of palladium being present.
Here is a recent article discussing the precious metals in MLCC's.
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