If you wanna make serious money selling games, then you need to learn the price trend for used and collectible items. Generally speaking, when a game launches it's worth about 80-95% of its MSRP once it's been opened. Fast forward about 6 months and maybe it's worth 25-50%. Move on to about 1-1.5 years and prices tend to bottom out unless it was a really limited release and has some cult following (things like Katamari for the Xbox are holding their value well for this reason). I just bought a PS3 (again) and 5 games. MGS for 10, Heavenly sword for 8, Uncharted 2 for 12, God of War 3 for 10, and Shadow of the Colossus for 30 (not an awesome deal on that one, but it's just gonna get more expensive at some point in the near future).
On the other end of that spectrum are games like COD, Forza, Sports titles, etc. which become nearly worthless once the sequel is released. This is the time to buy those releases if you have ANY interest in them, as they usually won't stay on shelves long with the price cuts that they get. I grabbed Forza 2 Platinum (with all the DLC on it) for .99 at Micro Center after F3 came out.
Special editions usually see a spike in value pretty shortly after release, especially if it's close to the holidays. This is because there's people that either wanted it but couldn't find it at release, or didn't realize that there was one at the time. I bought two copies of the special edition of Skyward Sword (yes, I was THAT guy), waited 2 months, then sold them both on Amazon for 149.99 a pop when I paid 70 for each online with free shipping and no sales tax. After fees and whatnot, I made about 100-110 bucks, which is an awesome return on a 2 month investment.
Once games get into that 10+ year-old range, then they become a lot harder to find. It's not to say that any 10 year title is worth money, but you better believe that if it was a relatively collectible title at its release, then it's gonna be really pricey at this point, ESPECIALLY if it has that cult-like status I mentioned before (see E.V.O. for the SNES).