Sold books online (one at a time, so only ones that could earn a lot)
Sold books in lots on Craigslist and in local groups
Donated them to my local library and Salvation Army
Given them to my kids' school
Given them to friends (after inquiring about their interest in the specific titles or authors)
Left them.in a box outside labeled "Free Please Take"
Placed them.in the free book thingy near my home where neighbors leave or take books (it looks sort of like an old time call box except with shelves and a glass door on top)