The simplest way to sell an item on Amazon is to list it in the Amazon Marketplace. There's no up front cost. With Amazon Marketplace, you take care of all shipping. Amazon sends you an alert when someone has bought your item, and you are responsible for sending it out with an invoice and such. Now note, the item MUST be in their system already, so this is good to do with books that have normal publishers lined up. You can sell an autographed version via the marketplace, and let them handle the larger number of sales.
If the book is NOT in their system, you need to sign up for a Marketplace Pro Merchant account, to add it in. This is $40/month so it's really only meant for larger companies.
Go to the Amazon.com website and log into your account. If you don't have an account, create one. This isn't a special account, it's the normal account you buy books with.
Once you're logged in, go to "Your Account" the link is in the top right of most pages. Make sure you're logged in. Then, at the bottom of the page, slick on "Sell on Amazon". The direct link is Sell On Amazon. The regular (free) account is the one they label "Sell a Little".
Now use their tools to search for the book you are wanting to list. When you find it, you can list the condition of the copy you want to sell. They show you right on the page the current list price for both used and new versions, so you can see how to price yours to get it to sell. That's really it!
Here's an example of an Amazon marketplace sale statement. For books, the Amazon commission rate is 15%.
Funds should appear in your account in approximately five banking days.
OK, now some discussion about the pricing of your books. Amazon is a bit intriguing when it comes to pricing. Let's take three of my books for an example. All were created in CreateSpace, which is Amazon's self-publishing wing. So all are paperbacks fully within the Amazon system. Let's start with Aspen Allegations. This paperback is 242 pages long and Amazon's minimum allowable price on that is $6.25. I set a list price of $6.99, and Amazon gives me a 44 cent royalty on each sale I make through them. It's listed on Amazon at $6.99 and on CreateSpace at $6.99.
Let's say I buy five copies of my book from within my CreateSpace interface, so at "author price". I get them for $3.75 apiece. Plus shipping it's $24.70 total, which works out to $4.94 each my cost to have them at my house. If I then list them at $6.98 on Amazon, to be the cheapest price, I can list them as "signed by the author". With each sale I make $2.04 directly. Also, Amazon gives me $3.99 to cover shipping. Media mail shipping on the book is $2.53 and let's figure another 47 cents for the envelope. So we get 99 cents from the shipping side. So that's a $3.03 profit we make on each book. Yes, of course there's the "pain" of having to ship it out yourself, but it is a special offer of a personalized copy which could easily get you more sales and fans.
So this time let's say I buy just three of them. That's $20.19 including shipping, so $6.73 apiece. I could list them at $9.99 and be way under Amazon's price and still make over $3 a book, plus the shipping money.
Amazon's minimum price on this is $10.63 - I set the list price at $10.99. Amazon will give me a royalty of $2.34 a copy with that. That's probably because the book is listing on Amazon at $14.99. Quite a lot for a romance novel. But when I order them within CreateSpace I can get them at $4.25 apiece. With shipping for 2 copies that's $12.68, or $6.34 each. So I could sell them at much less than $14.99 and still make the same royalty, and personalize them.
So what I suggest is, if you can afford it, to buy five copies of your book through your CreateSpace interface. Figure out your per-book price. See what Amazon says it'll give you for a royalty, then see what you could list it as. Remember to include the "profit" you make from their shipping amount. Then I highly suggest you list your book as a signed version in the marketplace. It gives another option for your readers, and you never know who might enjoy that!