How do markets, such as the sports trading card market, stay valued when the only reason that most of the cards have any value is because people want to resell them for a profit? In fact, from what I know, people who open retail boxes tend to lose the majority of their investments, even after reselling their sports cards.
In short, supply and demand which we will be covering soon. The cards stay valuable because people demand them - even if this demand is only due to an expectation of the price increasing in the future. This phenomenon is known as a bubble, and the term "greater fool theory" offers an apt name to a theory that describes some bubbles quite well.