"what are the chances that at that very second, somebody else wants to buy precisely 273 shares of Facebook stock? Probably zero" This is likely provably false.

"what are the chances that at that very second, somebody else wants to buy precisely 273 shares of Facebook stock? Probably zero" This is likely provably false. If you check the order books of Facebook, you almost certainly find buyers of Facebook during market hours that are not market makers. Would the spread be greater than 1 cent without the market markets, sure, but this statement is likely incorrect. And many orders can be filled partially.