Seller(s) of stock are not required to place shares on different exchanges. This is a side effect of the process of trading, as the NBBO price bounces around to

Seller(s) of stock are not required to place shares on different exchanges. This is a side effect of the process of trading, as the NBBO price bounces around to different venues. If I offer 100,000 AAPL at NBO at exchange X, and you are willing to purchase 100,000 AAPL at NBO, your buy order matches mine exactly and the transaction takes place. This is highly unlikely, of course, that an enormous stock transaction like this ($50M) will take place this way.