Quoted from CrazyLevi:Price guides are outdated and obsolete.
Your best bet is to simply pay close attention to the market here and come up with your own guidelines.
Asking advice here from the general community is still a good idea.
Asking a guy trying to buy a game from you is probably a bad idea.
Additional piece here, you can't even go off what pinside "suggests" when you look at a game's page. That is outdated. You need to look at the market listings and archived listings.