system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Selling shares on the stock market always works instantly. How is it possible that there is ALWAYS someone looking to buy what you have at market price?
gpt
Because when you say sell it at market price, you're saying 'Sell it for literally whatever someone is willing to pay' 

If there was literally no one willing to pay anything for it, the stock price would hit zero.