Kenslog.com

Internet resources
www.videogamepricecharts.com 

They are far and away the best and most accurate at retail pricing video games. There are others available but stick to this free resource.

Craigslist.org
This is the #1 place to collect. More and more people/gamers turn to Craigslist to sell there games. You can usually find great deals all over. Make people offers you are comfortable with the worst they can say is no. Always make sure that the games and systems work. If they are not willing to prove it then only pay site unseen prices. I have gone to the point of making a video on Youtube to prove it worked. Most Craigslist deals happen in grocery store parking lots as to offer protection to each party.

E-bay.com
As you may or may not know I am very Anti- E-bay. I do however have to include them in this conversation. E-bay is great for finding those rare and hard to find games and peripherals. Also an advantage of E-bay is you have 100% buyer protection. If your stuff arrives broken or it was misrepresented you have the ability to get your money back no matter what.
The downside of E-bay is you will pay top dollar for those hard to find games. Also you have to pay for shipping and handling as well. Always be sure to look at feedback ratings. I never buy from anyone below 97%.
Another drawback is that most sellers only accept the dreaded Paypal.

Amazon.com
Amazon is best known for selling books but they have expanded to sell just about everything it seems. You will find some good deals on this site occasionally when a seller doesn’t know what they have or seem motivated to sell their inventory. Always check feedback! Amazon seems to have some problems in eliminating there bad sellers. This has improved recently but you always need to protect yourself.

Cheapassgamer.com
Not only is this a great site to keep a list of your games but it is also a great site for trading games. You build a trade ranking with each successful trade. They have tons of active members so you should be able to find what you are looking for.

Independent retailers.

Sites like JJgames have a lot of inventory but should be avoided due to price.