gametz.com is a great, HUGE, original forum based site with the purpose of helping its members to buy and/or sell their used videogames.
The products are fully categorized by platform so that potential buyers can browse them in search for their favorite ones without effort and with little to none chances of making mistakes. The site offers reviews for some of the games and since this page is mainly a forum, you can take advantage of that and discuss your thoughts or ask experienced users about anything related to videogames or their respective sellers! If you are feeling nostalgic for that old game you used to play when you were a kid or want to buy some videogames at really good prices, then this should be the place to start. 
