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This site compares numerous book sale sites and gives you their cheapest price, including shipping!
BARNES AND NOBLE
THE PHILOSOPHER'S BOOKSTORE
Whoever owns this site (their identity isn't clear) has established this page in association with amazon.com. If you buy books from here, it appears that your money actually goes to amazon.com. It is likely that the owner of this page receives some sort of commission or compensation for each order placed from this page. There is a nice index of many major philosophers, and it is searchable.