Unless you happen to be an expert in book appraisal, it's a good idea to do some research on the value of your books before you set a price. You'll be kicking yourself later if you realize the book you sold for a measly dollar was actually worth much more. Some savvy booksellers will prowl around yard sales looking for just that -- hidden gems that are priced far below their real value. They'll quickly snatch these up and then sell them at a much higher price, profiting off of a mistake.
Luckily, in the Internet age, it's relatively easy to find the market price of your individual books. A quick search on Amazon.com, for instance, will give you an idea of what a particular used book is selling for. Chambal.com is another good resource. It lets you plug in the author and title or ISBN and will present you the lowest going prices across many different Web sites.
However, take note of the difference in various asking prices on each Web site. If there's a big gap between the prices, the lower-priced copies are probably in poor condition (or being sold by someone who doesn't know their worth) [source: Mould]. Also, be sure to check whether your copy is a first edition, special edition or latest edition, as these are often worth more.
Of course, if you have a vast library that you believe is very valuable, it might be worth it to pay for an appraisal.