I see it as a lost sale. That person who is playing the game and bought it used did not pay the dev team. They're playing the game and they payed the store directly.
i've heard all kinds of fluff and whatnot saying "the person might not have bought it at all" or "they might have spent that money on a movie or something instead" or "used games can generate fans and talk about a game that creates sales".
After all that is said and done (and is heresay anyway) what it all boils down to is that first used game sale. That sale, even if all those other things are correct, is still a lost sale.
In my eyes I think it's just as bad as piracy.
BUT, I'm not against used games. I own more than a few games I bought used. i just think anyone who tries to sugar coat it is lying to themselves.