I'll repeat myself: It's not like I sell my game to Gamestop and suddenly 5 used copies appear out of thin air. Like Hodgman said, it's one copy, one user. Only one person can use that disc and play that game at a time.
In fairness, you're right there is a difference. Used games are actually worse than piracy if we use money here.
New game sale: $50
Used game sales total: $175
Total sales: $225
Publisher profit: $25
Gamestop profit: $125
So Gamestop makes 5x as much as the publisher. Now let's imagine that instead of buying used games, the people pirate %50 of their games and buy 1/2 new.
New game sale: $150
Used game sales total: $0
Total sales: $150
Publisher profit: $75
Gamestop profit: $30
Well, I see how that sucks for Gamestop, but even people pirating %50 of their content is better than having a used market. The publisher makes 3x more if people pirate half of their games than if they get them used. Bear in mind, the average used game passes through six owners over the course of its life.
It's pretty darn difficult to defend used games.
There are so many things wrong with this comparison that I'm not even sure where to start, but on a fundamental level you're still ignoring the basic fact that, with used games, for each copy of the game, only one person can have it at a time. If we assume that games have, for instance, no replay value, then the comparison might start to make sense (but not your numbers; you've left enough variables undefined that they don't actually mean anything), but this is an incoherent assumption.
Here's another pointless comparison: games that can be played more than once are one million times worse than piracy.
I could pirate a $50 game and play it once. The publisher would (in some vague sense) be "losing" $50 dollars. But what if I bought a game only once and played it one million and one times? Then the publisher would "lose" $50 million dollars. The true criminals are those who dare to think that buying a game entitles them to play it more than once!
Now, this doesn't make any sense at all, because the price of games includes the assumption of being able to play a game more than once; a game that I could only play once after buying it would be a different (and inferior) product, so it's very likely not worth the same to any given consumer.
Just so with games that can be re-sold: with such games I have the option to a) keep a game and play it forever if they like it and b) give it or sell it to someone else if I don't want it anymore. This is something that I know in advance, when I'm paying for the game initially.
Remove any or both of these features and it's not the same product anymore, so we can't assume that it has the same value. Any economic comparison that doesn't factor this in doesn't actually accomplish anything.