Many stock prices actually go UP when all of them are sold, at least to a single buyer. Most famously in recent history, TWTR. The amount they go up even has a

Many stock prices actually go UP when all of them are sold, at least to a single buyer. Most famously in recent history, TWTR. The amount they go up even has a name on the balance sheet: Goodwill. The other big example is IPO's. Timed well, an IPO sees shares appreciate from their private valuation into a higher amount as they hit the public market. But yes, once already public, when being sold to multiple buyers, typically you're correct, a sudden increase in supply while demand stays constant does result in prices dropping.