The message you send to the customer is, I understand what you want, and if I dont have it, I know where to get it. The next time, they'll come ask you first, which is exactly what you want.
This gets to the heart of how one does business. Sure, you could be that guy who tries to scrounge every nickel, but you end up chasing nickels while alienating customers and fellow dealers. Taking a customer to a fellow dealer who has the material tends to work out well for everyone. And if that fellow dealer never reciprocates, it’s easy to forget what they’ve got in stock if anyone ever asks.