The stamp dealers made the price guide. The coin dealers made the price guide. Before the Internet, they created their own values. Once the truth came out about rarity and values, the market adjusted. Suddenly sheets and sheets of old mint stamps, were showing up, for under face value. I still use them for postage. Coins that were common are more common, and the prices dropped.

I don't like, the part about the dealers setting the values are having a personal interest in market values.

To be fair, this was NOT a one way slide. Some coins have appreciated in value and the true rarities have done just fine. But common coins that were over valued have seen their day. Grading and slabbing has created a new collectors market for the wealthy and high end collectors. People who collect as a hobby and for fun, can still enjoy things on a more social level, not so much about business and investment.