I have been selling books online for about 7 years with both Alibris and Amazon. Amazon has worked well for me due to the number of people who look to Amazon for their reading and research material. Alibris has done an excellent job of creating a market for rare books and supplies books to many of the online book sellers. Key to success with Amazon is understanding the market. Many books on Amazon are $.01 as dealers look to sell volume making their money on shipping.
Don't get discouraged as there are many more books you can sell for a market price where you can make money. Amazon lists a books popularity. It is found under a books details. This number is a key to success. Books that fall under 1,000,000 in popularity are good ones to sell. Next look at what Amazon is offering to pay for the book found on the right side. They will pay about 1/10 to 1/5 of current market price. If they are buying, it means the book has a good chance to sell Next check the new price vs the best used price. Is the used price close to the new price. What I mean example. New price $19.95 Best used price $10.95 This means the book has higher demand and sells for a higher price, a good book to list. Next look at how many books are for sell both used and new. Does Amazon show only a few books on hand. A good book to list. Understanding the books to list makes it more fun as you will find books will sell. Pricing, understanding the market in books, best books to sell, rare books, signed books are all other topics to explore once you have the basics. Good luck