I used to sell on half.com . My dad was a college professor and would get free sample textbooks from the publishers that wanted him to consider them for his classes. He'd give them to me to sell

But my most memorable sale was to see the name of a classmate on my half.com portal. I just handed him the book in class one day instead of having to take it to the post office.

I'd also buy deeply discounted old editions from the school bookstore to resell. They still had resale value because not all schools would update to the latest book right away.