If sellers can disappear, that's a problem. Amazon should be liable for ensuring sellers can be tracked down and prosecuted. In cases where they do manage to do that, though, should they still be on the hook?
I'm not sure there is any case where Amazon should not be liable. You buy something "fulfilled by Amazon" and at no point in the transaction is any sort of third party involved.
IMO, amazon would be better off moderating their platform.