I was a book seller on Amazon (as a business) for a number of years and made an average of $400 a month a number of years ago.
I would purchase books at garage sales, library's and many other places that sold used books. One key is to find books in very good to near new condition (no tears, writing or folded pages), . The books that garnered the most money were college text books (under 5 years old), non-fiction books, and technical volumes. Popular novel and novels in general are just about worthless. I once bought a book a my local library for a few dollars and sold it for close to $100. If you can find newer text books for $5 or less, you can sell them for anywhere between $35 to $60, sometime more.
There is a website (addall.com) that lists just about all books and what various sellers are asking for. As far as mailing, you use "Media Mail" (the cheapest way to go) or...if they want it fast, you would use "Priority Mail" which the customer would pay extra for. Mailing supplies can be purchased in bulk fairly cheaply. Get a postage service and a scale, so you can print your postage on your computer printer. When mailing, most of the time I would cover the book in a very thin foam material, and then use brown wrapping paper to mail it. Make sure the jacket cover is clean (you can purchase a cleaner that removes labels and glue).
