NSAV Holding Inc (NSAV)

[shotsky]

All trades are accomplished through market makers. There are dozens of market makers. A brokerage, such as etrade, accepts your order and passes it to the pool of market makers. The market makers don't know your name - they only see the order to buy or sell.
The market maker quotes a spread. The spread indicates at which price they will buy shares, and at which price they will sell them. But there is also a quantity for each, often shown as 1000x2000, meaning they will buy 1000 at that price or sell 2000 at that price. The reason the quantities are different is because they either have excess inventory and want to sell more than buy, or the opposite.
What you see on L2 is those quantities as tranches. If someone is selling 100K shares, it may take 100 tranches. You will see 100 sells, but that is only one person selling those 100K shares. After each tranche, the MM may change its spread. It could simply change the quantity so as to make fewer individual tranches or it could change the spread amounts. Computers figure out what is best for the MM and alter the spread as needed to provide the most profit to the MM.
MM's are not permitted to operate outside of their posted spread, and they are required to take the trade if within their spread.