You can sell books to Powell's too...

[jenk]

Books...are like lobster shells, we surround ourselves with 'em, then we grow out of 'em and leave 'em behind, as evidence of our earlier stages of development.
— Dorothy L. Sayers, The Unpleasantness at the Bellona Club, 1928

That particular observation comes from the mouth of Lord Peter as he's looking over a suspect's bookshelf. Some books we hang onto; I have books I've read til they're bleeding pages, their poor tired spines wilting away. I have others that seemed incredibly important at one time, but I never read them and generally forget I even have them.

I usually just take extra books down to Half-Price books because it's simpler than listing them on eBay or Amazon. I did run across another option on Unclutter - Powell's in Portland, via the web. You type in the ISBNs on their website; the site then tells you which books they're currently buying and makes a store-credit offer. If you accept the offer, you can box your books, print out a prepaid mailing label from Powell's, and mail the books off.

The drawbacks are that you get Powell's credit instead of cash and must have a Powell's account. But if you tend to buy from them anyway, that may not be much of a drawback. Alternatively, checking Powell's prices might useful to get an idea of what a secondhand bookstore will pay (compared to what used sellers are listing it for on eBay and Amazon and, oh, Powell's) and you can do it without leaving the house.