Final answer:
The individual is called a market maker. They are essential in financial markets for providing liquidity and ensuring there's always a buyer and seller for securities, making markets efficient.
Step-by-step explanation:
An individual who maintains an inventory in a security and stands ready to buy and sell at any time is a market maker. Market makers are entities in financial markets that provide liquidity by being ready to buy and sell securities at publicly quoted prices.
They play a crucial role in facilitating trading by ensuring that there is always a buyer and seller for securities, thereby reducing market volatility and making the markets more efficient.