I went to the annual Portland Retro Game Swap Meet (sponsored by SideQuest Games – a newer game shop that seems to land shockingly amazing and rare gaming gear on a regular basis – see the link) and picked up a few small things. I’m mostly interested in old PC stuff, so I’m not the direct target audience.
However, I did find folks referring to the following resources when valuing their games.
This site gives current and historical prices for almost every video game on every platform. They track and price loose, complete, new condition games. Fascinating site to see what’s hot, and what’s not.
WATA is one of the professional game grading services. You send your game and they pay for them to grade them. Prices range from $35 for a basic grading to well over $200 for rapid grading.