My wife and I used to have a lot more books too, but we have kind of been in a "simplify" mindset over the past decade or so. We tend to either sell books back to Thriftbooks or we give them away to other family members or friends. My brother in law also manages a resale shop that takes the profits and buys food to give to needy families, so we donate a lot of stuff to that too.
Oh that's very handy (and sweet!). It does feel like overcomplicating life a bit with all this stuff, doesn't it? :) I always wanted that kind of service - buy (used) books, then sell them back to us for store credit or such. Could keep me happy for months.
Yeah, we have a few small shops that do it like that. There is one fairly close in town that we can sell them back to if we want. We haven't visited it yet though.