In certain ways it actual helps, when people know a game can be traded back in for $20 if they don't like it or sold for more it gives the game a certain inherent value in the consumers mind. If videogames were more like magazines where used copies are worthless people probably wouldn't be as willing to spend $60 on each title.
Also if used games are bad for the market I would suspect rentals are even worse since far more people play a rental game than most used and resold games. Yet most game companies allow rentals.







