Authors and independent publishers are sometimes upset to find that online bookstores like Amazon.com are selling their book at a discount from the list price (the price printed on the book).
For books sold and shipped by Amazon, the fact that Amazon has chosen to sell at a discount to list price has no effect on the amount of money that the author or publisher earns – they still receive their standard payment from their publisher, distributor or wholesaler.
One positive effect of used book sales is that it gets your book into the hands of a wider readership – these readers may tell others about your book, buy new copies of your other books, or purchase other products and services from you.
See this article for a explanation of how the Amazon Marketplace works and how you can list products for sale there.
For an explanation of reseller discounts and the role of distributors, wholesalers and retailers in the book trade, see this article.