I wouldn't pay for it, Okie. Not yet anyway. Good chance Joe actually still has them, and if he doesn't have them, then good chance he can tell you who does.
Your Paypal payment won't be protected if the seller closes his account right after he receives the money. By the time you have evidence that the marble isn't arriving, it could be too late.
I'd talk to Joe before I sent payment. If you've already sent payment, you could file a protest right now. Paypal suggests you wait, but last I heard they don't actually require it.