Buy underpriced used books. Outfit a phone or PDA (personal data assistant) with a scanner, scan the ISBN numbers of books at used bookstores and thrift shops, and compare the asking prices with what the books are selling for on a site like Amazon. Whenever you get a good hit (which won’t be often but, since the process is fast, won’t take long, either), buy the book and resell it online. Be discrete about this, as the store managers probably won’t like what you’re doing.
