If you are in your local public library right now, how about approaching one of the librarians and asking if they have a books for sale area or shelf.
Many public libraries sell their less popular books. This process is called weeding within the library service. Like a weed in a garden a book which stays firmly on the shelf in a public library serves no good purpose. Like a weed in a garden it steals a vital resource, space on the shelf, instead of space in the vegetable plot or flower bed.
These weeds do serve one purpose for a library, they can sell them at a much reduced price and recoup some cash to fund buying of new and hopefully more popular books.
Libraries "weed" between one fifth and one third of their books in any one year. This means that millions of books are available across the US everyday in city public libraries.
You can pick up books for a few cents for childrens books or a dollar or two for a bestseller.
This little goldmine can be yours for the asking. Invest a few dollars in some of these weeded books and take a trip to your computer and EBAY or one of the many online auction sites.
Library books carry library marks, stamps and you may find a torn page or children's books have crayon marks. It's a risk.
Leave torn and crayon marked books on the shelf but books in reasonable condition even with library stamps and such can return a reasonable profit on EBAY.
For a small EBAY fee and the price of postage and packing you can maybe make 50% or more on some of the more popular authors books. Fans of authors will buy books that are out of print to read. A collector may buy even a marked first edition if it is all that is available for the price.
You won't get hundreds or thousands of dollars for a first edition book with library marks, but if you get $10 for a book you paid $2 for, then don't fuss. Cash is cash and we want to make lots of little profits like that one.
For other ideas as to where to sell old library books see my article at Yahoo! Voices