"You" vs. "other people". In bubbles, the investors just trade the shares among each other. In a Ponzi scheme, there is a central party that refunds investors

"You" vs. "other people". In bubbles, the investors just trade the shares among each other. In a Ponzi scheme, there is a central party that refunds investors on demand, and does so with money from new investors. A Ponzi scheme is always a fraud, frothy valuations in a bubble might indicate market manipulation, but not _necessarily_ fraud.