I know a lot of people are thinking about weekend deals, and how people could make back money by buying sale games. But that isn't an issue. There is information on the servers of how much you paid for a game. You couldn't wait for a used game to appreciate, because it will always be of equal or lesser value. No, there are too many OTHER inescapable reasons why this idea will never work.
Alright, Valve is awesome. They know that excellent customer service gets them business. They know that free content generates good will, and trust in new properties. They want to make gaming better. But however awesome they are, they are also still a business. They will certainly provide you with free content when its feasible, but they arn't going to, essentially, give you free money. Used games are a very different beast: Give a physical copy to someone, and they can do whatever the hell they want with it, which leads to a secondary market. That's just beyond control, and companies are trying to reign the resellers back in as it stands. But if Valve lets you turn in games that you have already played a bunch of...how do you not go bankrupt? The developer who made the game being traded in isn't going to pay Valve anything. They have their money already, and if a developer suddenly started losing money after people stopped being interested in the game, then they could simply not be profitable. At least, not without INSANE price hikes, mitigating the loss in sales by drastically reduced production value. The developers of the new game arn't going to pay anything. They arn't going to accept part ownership of another developers game as payment, and they arn't going to give away there games for free. Which means that Valve is picking up the tab. Meaning that every time they sell a game that is going to be turned in, all they are making for profit is the convenience fee. When you trade in a game, the people you are trading to get something physical to sell, and make more money: What do Valve get? Their data back? No, it pure, unmitigated lost profit. Valves a sweet company, they're not stupid. Barring some crazy business model that allows them to make more money by making a fraction of the profit they once made from selling games, this just can't happen. It doesn't make mathematical sense.