Its a difference but its still not something you can prove. Say customer A usually buys used games around launch for maybe 15$ cheaper, who are you to say that he/she won't buy it new for $50/$60 if its the only thing available?
Most games these days have matchmaking or at least stat tracking and those cost server and infrastructure to run. There are plenty of ways publishers can lose money on a used sale and those occur often.
http://www.thesixthaxis.com/2011/10/17/interview-naughty-dog-on-uncharted-3/
I'm not talking about proving anything, I am trying to demonstrate there is a difference between getting zero revenue from a sale, and losing money due to that sale.