For me unless it was something like a 1st edition Deities & Demigods (and what is?), I am like Dyson Logos and look for someone to give it who might need it/like it more than me rather than try to sell it. The value I get from helping out another gamer is greater to me than any cash (tho I also understand that is a privileged position). I tend to keep stuff but I will give away stuff (like I never run the Realms, so I give away my Realms books unless they are generic enough for me to use) every couple of years.
I usually also give it away with one stipulation: If it turns out you don't want it/can't use it, please either pass it on to someone who does or give it back to me and I will try to find such a person. That is how I gave away my entire collection of Dragon Magazines, but then got them all back 10 years later!