Two Customers, Both Sides

Each fill earns the half-spread relative to fair value.

The half-spread is $0.20$ per unit on each side.

1. 1. Customer buy

   You sell $10$ at $50.20$, earning $0.20$ each above fair value: $+2$.

2. 2. Customer sell

   You buy $10$ at $49.80$, $0.20$ below fair value: $+2$.

3. 3. Total

   Inventory nets to zero and the P&L is $+4$.

4. 4. Key idea

   Balanced two-sided flow is the ideal for a market maker: spread is earned with no inventory risk.

Answer: 4