Lend them to friends when they visit, give them to a charity shop - that way someone else will enjoy them too. Why would you need to get money for them?
There's no need to throw a good book away, or leave it collecting dust on your shelves (which is close to the same thing). Most books nowadays won't last longer than a human lifetime due to the perfect binding and terrible paper, so you're not keeping them for posterity by hoarding them.
I'm with you on goodwill and lending, or giving. I found that most folks don't give back when I lend so I don't create the expectation that they should.
There's no need to throw a good book away, or leave it collecting dust on your shelves (which is close to the same thing). Most books nowadays won't last longer than a human lifetime due to the perfect binding and terrible paper, so you're not keeping them for posterity by hoarding them.