Executing the Trade
All sales of stocks must take place at the stock exchange where the stock is "listed". Some stocks are listed on the New York Stock Exchange (typically abbreviated as NYSE). Others (especially technology stocks) are listed on the NASDAQ exchange. These exchanges handle an enormous number of trades every day - the New York Stock Exchange has sometimes handled the exchange over 2 BILLION shares in a single day! The number of shares exchanged is called volume.
Prices are always changing. It isn't uncommon for price to change between the time a buy or sell order is requested and the sale or purchase of the stock is actually made, though the difference is often small.
In our example, the price hasn't changed. Drew sells his 500 shares for $20 per share, and Meghan buys her 500 shares for the same price.