I've done this a few times. In college, I went on a long book-buying binge. It lasted several years. It was something of an obsession. I bought all kinds of "classic" books very cheaply - at used book stores, garage sales, book fairs, you name it. A couple of years after college, I had about 4,000 books. At some point, I needed to get rid of them, and I needed money.
I loaded up my parents' van with several hundred books. I took them to John King Books, the largest used book store in Detroit, and one of the largest in the world. It's absolutely huge - 4 floors, and each floor is easily 5,000 feet. It dwarfs any Borders or Barnes & Noble. Some old guy with a thick white beard offered me $125 for the lot. I turned him down, because I was sure I could get more somewhere else.
I went to another used book store and sold a chunk for $100. Went to another and sold a chunk of the remainder for $20. Went to another and finally got rid of the last few for $3. Total: $123. It was a good lesson in negotiating and efficiency.
Over the years, I've occasionally gone through my remaining books and performed a purge. There are always a few moments of "No, I really don't need this." The last time was a few months ago. I still had several boxes of books in my parents' basement. This time, I took the load to John King, and they didn't want them. Wouldn't pay me for them. The best they could do was store credit. $70. I tried to give that away to my parents or a friend, but no one would take it. I've still got it. I went to another book store with the ones that John King wouldn't even exchange for store credit, and got $17 cash. For books that were worth probably $250-$300 new.
What we're seeing here is the blessing and curse of efficiency. Over the course of the 20th century, it became easier and easier to print and distribute books. As they became cheaper, it got easier to collect them. But as the cost decreased, so did the value. One reason I feel comfortable getting rid of books is that I know I can almost always find any particular book again.
In some respects, it's somehow sad to see books lose their value. But there are upsides. Given that they are now so cheap as to be almost worthless, it's easy to give them away. There are still many places on this planet that could use more books - small town libraries, prisons, even developing countries. There are programs that send books from our overstuffed personal libraries and bookstores to these places.
Of course, the downside of books being so easy to make is that it takes a certain amount of resources to make them. The Kindle and its ilk are rendering physical books less and less relevant. They're also letting us read books with far less use of resources.
Physical books have lost their value. But knowledge and writing have not.