Donate and tax deductions easier (plus the feeling that the book will be appreciated by new reader) and more lucrative—unless you file 1040EZ and don't do deductions.
Goodwill in conjunction with IRS offers this guide to setting
value of donations guide which says hardcovers are $1.79, Soft Cover 89¢, children's 59¢.
It can be worth turning a recently bought amazon book back to them via their resell program or checking the price on a very old out of print book in case you do have something collectible; but, otherwise selling a book on ebay, half.com, amazon.com or other sites just puts you in competition with bigger fish that can afford to list book at 1¢-$99¢, plus hassle of shipping, plus possibly a buyer who says there's a problem where you have to refund what you did get.
I'd rather count up number of hardcover, softcover and children's books and give to a charity or shelter all in one trip than ship each one.