Other than giving them away to family, friends, and charitable organizations, I buy used books at thrift stores, library book sales, and church rummage sales. My average price per book is about 25 cents. I then turn around and sell these books on half.com for more than I purchased. Sometimes it's a lot more, but usually only a dollar more than I paid. The customer has to pay for shipping as well. when I take the book to the post office, instead of buying postage, I use the stamps.
For example, I bought a book for 25 cents, I sell it for a dollar and also get 2.64 in shipping cost. I wrap up the book, take it to the post office and use the stamps. So, my profit is the 75 cents and the 2.64. It is not a lot of money, but it is how I use up my extra stamps. I also ship these books when doing regular box and DIM box shops, making 2.64 profit from the shipping as well.