Where do you buy your oldie, but favorite video games? I seem to have the best luck through local Craigslist listings, (price wise), but for select games I have to use eBay. Using Ebay seems to be more on the pricey side due to more knowledgeable people selling games. I am trying to find Fire Emblem for the GameCube and it sells for almost $150 on Ebay. Way too much for my budget!
The truth is I've had the best of luck at thrift stores. By luck I mean in terms of the price. they very seldom look up retro gaming systems or games before putting them on a shelf. They become the same price regardless of whether it's Panic Restaurant for NES or M&M Racing for Wii they will usually run about 5-10 dollars.
You could try your luck at pawnshops or secondhand stores, occasionally you might stumble across a rare gem. Also, yard sales could work depending on your neighborhood.
eBay is almost the worst way to buy retro games, since people are driving the prices up artificially. Your best bet is to find a specialized retro game store nearby, most people there actually know the worth of their games and won't try to rip you off.
We buy from our local Disc Replay! It is a resale shop that sells movies, vintage video games, and newer stuff as well. They take trade-ins for cash or credit as well. They are a chain but there are not too many of them. I also like yard sales and church rummage sales.