I'd like to say first that "fair market value" is whatever the market says it is.
Usually, yes. But when the market is being intentionally manipulated, it's not. So in this case, the price does deviate a bit from fair market value. But the person who made it do that loses money. You can always manipulate prices to be unfair if you're willing to burn money to do so.
For example, you buy up lots of oil and watch the price go up. But what do you do with all that oil when the price goes down? As soon as you stop buying, the price rockets down. And you can't be selling while you're buying.
So I sell my shares at $18.75 (your example below) then and follow the price back down to $17.
The price will drop so fast, it's very unlikely you'll make back your money. But if you did, it's because some suckers bought at an unfairly high price. You can always make money if there are suckers.
It's not a crazy assumption to suggest that "on my way up to $20" I've actually paid on average about $18.5 for those shares. So I've got about 5k shares at $18.5 and another 2k at $17.
If I sell them all at $18.5 I'm still very much in the green.
You won't be because the volume as the price drops will be very, very low. Whereas the volume as the price goes up will be very, very high.
When the price is above its medium-term average, people are anxious to sell. They know they're getting a probable windfall. So it takes a lot of shares to push the price up. The net effect is that in the very short term, the further the price is from the medium-term average, the more it wants to go back to the middle and the harder it is to keep pushing it. Sell orders will keep flooding in and buy orders will not. So the part you are paying for is "uphill" and the part you are getting paid for is "downhill".