Generally, an individual who has purchased a copyrighted book has the right to re-sell that book under the "first sale doctrine" of the Copyright Act. The Supreme Court may soon refine the boundaries of the first sale doctrine, limiting it to purchases made within the US. Thai student resold textbooks originally purchased in Thailand. A publisher recently brought suit (Kirtsaeng v. Wiley & Sons) after finding that a Thai student had been reselling textbooks purchased in Thailand. The publisher claims that the books were illegally imported and are therefore not entitled to the protection of the first sale doctrine.
UPDATE:
Supreme Court recently held that US Copyright law does not prohibit unauthorized imports and sales of copyrighted material manufactured outside the US.