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Why was my sell order with an order price lower than the best ask price not filled

Due to the trading rules for the dealers and market makers in the US, a small number of sell orders with an order price lower than the best ask price cannot be filled. The reasons for this are as below.


1. Quote Delay

Under the US stock trading system, most transactions are entered into through dealers and market makers. They are required to notify the public within 90 seconds after each deal is made. In this case, the information we see on all real-time market quotes systems is delayed.


2. The definition of "lot"

In the US stock market, the unit for buying/selling shares is one. However, in order to reduce the transaction cost and lower the risk for long and short positions, dealers and stockholders will trade in unit of 100. In this understanding, the unspecified one "lot" is 100 shares. If you place orders not in multiples of 100, you may need to wait for a while until they are, combined with orders placed by other investors, in multiples of 100.


3. Short selling

The pool for stock shorting may out of stocks when short orders are placed; however, these orders will not be rejected immediately. If the pool has incoming stocks with proper price, they will be filled; if the pool does not have any incoming stocks by market close or the incoming stocks’ price is out of range, these orders will be cancelled.

Moomoo is a professional trading app offered by Moomoo Technologies Inc. In the U.S., investment products and services on moomoo are offered by Moomoo Financial Inc., Member FINRA/SIPC.

Any illustrations, scenarios, or specific securities referenced herein are strictly for illustrative purposes. Past investment performance does not guarantee future results. Investing involves risk and the potential to lose principal.