I buy most of my curriculum used online. The places I always check are e-bay and homeschoolclassifieds.com. Homeschoolclassifieds.com you set the price, many include ppd after the price to indicate price includes postage. I like this better than e-bay as I know the price up front. Many people also go to vegsource.com/homeschool, too. I personally am not as fond of it but LOTS of people use it.
Here are some others:
homeschoolbid.com
home-school.com/forum/trades
theswap.com
HTH Jen