You probably don't realize this, but most other goods have a certain wear and tear that video games (and other digital media) don't have. A used car, or TV is not going to be as good as the day it was bought due to everyday wear and tear of the buttons, tubes, engine, and just about every other component. Unlike other goods a video game isn't going break due to it being used therefore a used game is pretty much just as good as a new one (assuming it's fully working when you get it). If this applied to cars, or TVs or any other industry they'd definitely be complaining.
Now let's look at the numbers. Currently the video game industry does about $10billion worth of sales per year. Now gamestop's used game sales are about $2billion, and that's just one company. No other industry has 20% of their sales matched by a used market let alone a used market from a single company. With companies like Amazon, and Best Buy are looking at ways to break into the used game market it's pretty clear the used game market is going to be growing, and it's only going to hurt the companies making the games.
Now I'm not expecting the game industry to completely dissolve used games (I mean look how well that worked for the PC), but I think there should be some small re-licensing fees associated with used game sales that will help fund new a better games in the future.
Hypothetically let's say a company that does over $1 million in used media sales pays a 10% used tax on the sale of each title to the publishers that own the rights to said media. If gamestop passes the tax onto those selling the games (I should also mention a lot of games sold to gamestop are stolen) than they can profit even more.
For example let's say they buy a game for $10 with the intention of selling it for $30. If they apply the 10% tax to the seller that would knock the price down to 7$ since the tax is paid at the resale, not the trade in. Now as time goes on the game doesn't sell and the game price drops to $20. If gamespot sells the game for $20 they are paying $1 less tax than originally planed and technically profiting. Now you're probably saying they would of made more had they sold it for $30, but it doesn't work that way tax or no tax.