Masada:
I've never sold books through internet because I'm too lazy. If the book is interesting enough, you'll probably make more money without middle hand.
Here's what I would do:
1. Take old and rare books to fancy people's bookstore. Usually situated in an old and distinguished part of the city, and they play classical music in the store. There's of course no "secondhand" in the title of the store, but rather something like "antique books"
2. Books that are valuable due their special knowledge: put a list on the noticeboard of the local university. Secondhand stores usually lack the knowledge to appreciate these, unless they are really specialized to the subject (only in bigger cities).
3. Literature (if not rare) is best appreciated in middle priced stores. Expensive places don't sell books for reading but for showing, and can refuse good popular books if they only have wrong kind of cover for example.
2&3. If there's university flea markets or something like that, try it. There's probably people who are interested about the subjects you are (have been), and who like to read literature. Price literature moderately there. Typically 2nd hand book store gives you less than 50% of their selling price, so if you sell it for something like 60-75%, it's a good deal for you and the buyer. This is also good place to get rid of books no one would pay anything for.