A simple Google search finds international editions of most major economics texts on E-bay. They are typically shipped from within the U.S.A. Prices, including shipping, range from $25 to $60. Although publishers attempt to discourage reselling of international editions, the practice seems to be entirely legal. An interesting discussion of the relevant laws is found at http://answers.google.com/answers/threadview?id=295219
Most bookstores will not buy back used copies of international editions. That is a disadvantage in all but the last year of an edition, when bookstores will not buy back used copies of the domestic edition, either.
In the second year of an edition, a student may be able to buy a used text at 75% of the new price and resell it at 50%. If so, the net cost of owning a used copy of a $160 textbook for a year is only $40. A student who buys an international edition for $40 gets a slightly lower quality book, but can keep it for reference.