Would most of the people who choose to buy and play new games continue to spend the same amount on new games if they couldn't resell them when finished? Probably not.
What you're arguing here doesn't necessarily imply that developers would get more sales, because if a person is reselling their games, there's also a good chance they may actually be buying a used game. A used game sale may mean either a trade-in for a new one, an old one, or nothing at all. Logically, then there's fewer new games purchased after a trade-in than there are trade-ins, so trade-ins are not 100% helpful. Really, it's a case where you'd have to do a study to see whether used game sales actually increase sales enough for developers to be profiting from them. I don't think you can make an anecdotal conclusion on that issue. In any case, I think we'll see this issue go away with digital sales and online only games. There's just not much incentive for a game developer to allow used game sales if they can prevent it.