I often sell CDs and DVDs. I have written about my massive purging of thousands of CDs. Even after that I continue to to sell and buy. I usually take a big bag and get around $200 in cash, or more in trade. I tend to buy titles that most little independent stores want to buy back (no Keith Urban in my collection). It's a good deal - better than throwing them away.
I used to sell / buy books in Portland (Powell's) and NYC (the Strand) and always did pretty well. I use to help out some friends of friends in publishing that needed favors and they thanked me by sending me boxes of books. Most of these books I'd never read so I just went and sold them - I always took the trade at bookstores. A bag of books would get me more than $90 in trade which is not bad... used books can be as little as $3-5. Sell some, buy some. The best part is they usually took most, in not all, of what I brought to sell.
Last weekend I went to sell books in Chicago for the first time. I went to the Powell's on the Northside since it is close to my house, and it's Powell's... I love the Powell's in Portland. What I quickly learned is... Powell's in Chicago blows ass. I took two canvas bags full of really hip / cool fiction and cook books and they took maybe six of them and gave me $10. $10! They offered $17 in trade so I had to go with the trade which I spent on some Jane Austen books for LP. The trip was not even worth the gas I used. Grrrrr. Wish I could ship my books to Powell's in Portland and have a friend sell them (I have friends who work there)... but after paying for shipping it is kind of pointless.
I guess I should stop buying and just go get my books and the library. I am thinking about donating the books I do not want to the library... maybe someone there will want them.