I homeschooled for 19 years, and there are a number of online swap boards for school books. My favorite was at Vegsource: http://vegsource.com/homeschool/
You can check out the ads and contact people that have books you're interested in. Sometimes you can dicker on the price. Going rate for a good-condition used book is half the retail price.
Also check e-Bay, but a lot of homeschoolers quit using e-Bay because they made a rule that you can't sell teacher's editions. And in some of the higher level math and science, you've just got to have the teacher's edition.
He hath shewed thee, O man, what is good; and what doth the LORD require of thee, but to do justly, and to love mercy, and to walk humbly with thy God? ~ Micah 6:8