Inventory After a Customer Buy

When a customer buys, who is selling?

Dealer inventory falls when the dealer sells.

1. 1. Identify the side

   A customer buy means you sell $80$ units at the ask of $12.30$.

2. 2. Update inventory

   Selling reduces your position by $80$, so the change is $-80$ units.

3. 3. Key idea

   Always translate customer direction into dealer direction before computing inventory or P&L.

Answer: -80