There is another factor - the arrival of the indistinguishable fake. There's been one rather crappy attempt complete with an unconvincing sticker on the back but a properly convincing one is not that hard to achieve so I'm surprised it hasn't happened yet.
Once it does that's going to be a huuuge problem. Not many coins will have unimpeachable provenance but even those that do may have buyers doubting in that scenario.
Wrong. Buy from trusted buyers, sell to anyone. There is no such thing as an indistinguishable fake. If people really wanted to, this issue could be prevented before it even starts. However, I highly doubt anyone wants to maintain a database of anonymous ownership for free (I know how someone could make it: untraceable, unlinkable, private; don't waste your breath). In addition to, of course, people not wanting to cooperate/contribute for any reason (even if it is something idiotic such as laziness).