Quick definition Inventory risk is the risk that a trader or market maker faces from holding positions in securities. If you are long inventory and the price falls, you lose money. If you are short inventory and the price rises, you lose money. What it is Market makers must hold inventory to provide liquidity. They buy shares at the bid price and sell them at the ask price. Until they sell that inventory, they are exposed to price moves against their position. Inventory risk motivates market makers to adjust their quotes. If a market maker accumulates long inventory (more buy orders than sell orders), they widen their ask prices to encourage selling. If they accumulate short inventory, they widen their bid prices to encourage buying. Why it matters Inventory risk is the core reason market makers need compensation. Without compensation for inventory risk, market makers would not quote prices and liquidity would disappear. Inventory risk also affects market dynamics. When inventory risk is high (volatile markets, uncertain conditions), market makers widen spreads and liquidity dries up. When inventory risk is low, market makers tighten spreads. Inventory versus adverse selection Inventory risk and adverse selection are two separate risks facing market makers. Inventory risk is the risk from holding positions. Adverse selection is the risk from trading with informed traders. Both widen spreads. Practical example A market maker quotes bid 100.00 / ask 100.10. A buyer hits the ask and buys 50,000 shares at 100.10. The market maker is now long 50,000 shares, facing inventory risk. If the price immediately falls to 99.50, the market maker has lost 30,000 dollars on the position. To manage this risk, the market maker lowers their bid to 99.80 and ask to 99.90, hoping to sell the inventory quickly and close the position. Hedging inventory Market makers use several strategies to manage inventory risk. They can hedge by taking short positions, by using derivatives, or by adjusting quotes to encourage buying or selling to offset the position. See also - Market Maker - Adverse Selection - Risk Management - Hedging
