Science textbooks are always expensive (think $100+ Resnik Halliday et al).

I went to Michigan State for my Masters in Physics and was scheduled to go back to India during my first summer. So a month before, I researched for the top 10 selling books on Amazon, priced above $100.

I came back to India and bought a large size suitcase. The biggest possible and allowed.
Went to the Indian wholesale book market (one of Asia's largest - called Nai Sarak) and bought shit loads of the same Top 10 books. They retailed in India around $10-15 and were Asian paperback versions.
Loaded my suitcase. It weighed as much as 2 with clothes!

Reached Michigan and listed them on Amazon reseller and ebay (used text sections) for $50-60.

Sold all of them (around 35 books if I recall correctly) in less than 3 days.

Net profit of $1200 after all fees, with an upfront investment of $500

1. eBay top rating (awesome seller A++++++++++)
2. My first ever laptop
3. Next year's ticket back to India!

Fun times! :)