My imaginary pop machine has a second parasitic vending machine next to it that can automatically buy $0.50+2h beverages, and sell acquired stock at a variable instant price, dependent on its internal prediction model and the number of currently available bottles.

That parasitic vending machine has another parasitic vending machine next to it, which has an ever better prediction model, such that it buys from the other two machines only when it knows that the second one hasn't bought enough to meet future instant demand. For instance, perhaps it knows the schedule for a nearby convention, when the other parasite only knows average daily purchase volume. So 2 hours from the end of the headline event, the third machine puts in a big order from the first, and then two hours before the end of the expected surge, buys out the second machine so that it can't undercut using its available stock. It then sells out at a higher price and waits for the next opportunity.

So the only time anyone ever sells a drink for $2 is when an unexpected busload of thirsty tourists drops by and buys out the entire stock of cheap, instant beverages.

In your example, you have also forced my hand. What happens if I instead buy a 2 hour ticket, right in front of the guy, and ask what his price would now be to trade my ticket for his?