I'm on a couple of Library Boards, one of which runs Quarterly Book Sales to support the library. I've been doing this for about a decade. Here's what I know. Books don't sell at garage sales. Listing anything with a value of $10 or less on line probably isn't worth your time and effort. Book buyers are notoriously picky, and Amazon slaughters the seller in favor of the buyer. Know this.
If you itemize your taxes, donate the books to your library and take the write-off. Best and easiest bang for your buck. Next, consider creating your own branch of a "Little Library". Stock it yourself for a few years or more. It's fun, cheap, easy, good karma, and you may make a few new friends. There are two on my street and I stock them both as needed. I also tend to accumulate Chamomile Tea, which I don't like. I put the sachets in cute little box and tucked it into the corner of one of the boxes, a trick I learned from Katy at The Non-Consumer Advocate.
Finally, stop buying new books! Or be like CheapScholar and support your Friends Group's book and bag sales. By the time the bag sale begins, we are so over it, we'll practically shove the books in your bag for you so we don't have to pack it up the next day.
If you Fifty..., CheapScholar, or anyone else reading this is serious about selling books, another great way to obtain them is by joining a Friend's Group and helping with book sale set-ups. Our volunteers can buy anything they like before the sale for $1.00. On the take-down day after the sale, they can HAVE anything they want. Just sayin'. There's not a library on the planet that can't use more volunteers. If we both make money, everybody's hap-py.
Get a library card. It's the best. I think a library card should be required ID at all MMM meet-ups, camps, and all other functions.