Can someone explain how dealers make money?

[NYRolexx]

I am very curious as to how these dealers make money on watches.

How do guys like DavidSW make money? The prices they have are already very good. How are these guys buying them that much lower to turn a profit?

I understand people trade in watches a little under market value to avoid the hassle of selling but are that many people doing it?

I understand someone doing it as a hobby or to buy a few extra watches a year but how do people run businesses doing it? Hard to wrap my head around it.

Completely understand if no one wants to give up any trade secrets, I'm just very curious