What's involved: You'll spend a few hours a week going to bookstores and finding books that are in hot categories. As you grow you can use a "scouting" program on your cell phone to check what a book sells for on Amazon before buying it for your inventory. Books are bought for $1 or less.
You'll post books on Amazon and other book marketplaces so there is no need to find customers yourself.
Use a service like www.fillz.com to automate the selling process and sell your books on five or more markets. When a book sells it gets removed automatically so it doesn't sell twice.
Once the book sells you'll print postage on your computer or take the packaged books to the post office. Most booksellers use inexpensive envelopes and the Amazon shipping allowance normally covers postage plus materials with a small profit left over.
Business Costs: $80 for your initial book inventory and $20 for supplies.
Growth: When you grow you will want to get an Amazon Pro account for $60 a month to automate your selling tasks. You'll also want to consider a service like Endicia or Stamps.com to print postage for $16 a month.
Cost Saving Ideas: Print postage on plain paper and tape to the packaging so you don't have to buy labels. Start your initial inventory with books you already have, but want to get rid of.
Bonus: You can work from home and set your own hours. If you enjoy visiting bookstores this could be a great opportunity for you.