Good Evening (from Italy)
This site is good: http://www.bookdepository.co.uk/ they do free, worldwide delivery and you can pay in most currencies. I check between here and Amazon for most books and go for whatever is cheapest.
Craigslist. Tons of textbooks for sale in college towns.
A lot of students I see these days seem to get pdf's of textbooks that they probably get pirated online.
Textbook and cheap belong only in the same sentence when separated by "not".
The value of saving all your old college textbooks is debatable. For one thing, the college textbooks as produced, are not intended to last any length of time...the publishers rely on low quality and new editions every few years to keep the used book trade from gutting profit margins.
The problem you will find with buying textbooks online, especially used textbooks is that your instructors (no matter what academic "rank" or title) will be using the most recent edition, and often there are just enough differences to affect the utility of the text.
After a few years, my saved textbooks collection amounted to a few books containing formulae & tables that I used just enough to justify keeping the references.
I see what you are saying Native Son. At my school sometimes the rentals are actually more expensive than the used, and if I get books that I can keep I can check off stuff in them or circle stuff that is vitals to studying for test's.
These are the sites my students use if they don't use Amazon. You might check the list here: Top 20 Book Sites