The practice of buying a stock, promoting that stock, and then selling shares as the price rises is called a "pump-and-dump" scam. It's an age-old technique used by market manipulators, and it's illegal when done when in the stock market. The cryptocurrency market, on the other hand, is unregulated and more like the Wild West than a regulated market. Not surprisingly, people have been endorsing cryptocurrencies that they own without sharing the conflicts of interests that may exist. This New York Times article explores the murky world of celebrity crypto endorsements.