Say, to make things simple, a publisher knows that they've sold 15,000 copies of a game at $50, for a $30 profit. They have 20,000 players who've registered to use their matchmaking system. Thanks to an online "check in" system they've implemented, they know that everyone in their player pop had a legally acquired copy at the time they registered (no pirates), so they know that 25% of their legal playerbase bought used/rented/acquired a legal copy in some way that didn't give them money directly.
So we have 5,000 players who haven't bought anything directly yet, but are still invested in the game. In fact, these 5,000 players bought their games via Gamestop from the 5,000 "new" players who bought the game for $20 and then got tired of having it. So instead of the active player base going down to 10,000 from 15,000, the addition of the secondary market maintains the pop at 15,000.
Now the pub has released their first DLC for $10. 75% of their remaining original base chooses to buy it (7,500). Let's say that the second-hand users are, for the most part, thrifty folks with not a lot of immediately available budget, and only 50% of them buy the DLC (2,500).
So:
1) 7,500 "new" players who are worth $40 each ($30 new + $10 DLC)
2) 7,500 "new" players who are worth $30 each (new only), 5,000 who left
3) 2,500 "used" players who are worth $10 each (used only)
4) 2,500 "used" players who are worth nothing except their company in helping maintain the community
$550,000 in revenue
vs
Only the first 2, which comes to $525,000. And a smaller playerbase for the future.
Now, there are definitely assumptions, to be sure. Conversion rates, sales rates, abandon rates, are all assumed. We're assuming no "new" batch of customers, that some undefinable proportion of the "used" group wouldn't bite the bullet and buy "new", and that the pub doesn't sell advertising at a very profitable margin within the game. And there's no use-cost per registered user.
But the point is, it can be very, very quantifiable, and can be directly tied to direct revenue to the publisher. It really just depends on what the pub is willing to track, and how they're willing to exploit behavior to monetize it.