How would Amazon feel if I published an ebook on their site for, say, $5, and then offered the same book on my personal website for $4.50?
The idea is to allow Amazon to market my books and make my pen name well known, and eventually switch to a model that allows me 100% of each sale instead of 70%.
If you're actually publishing through Amazon, you sign over certain rights, such as the ability to sell or distribute the book through a separate service. This will be spelled out in the publishing contract. There are usually stipulations on when that contract expires (x amount of time, if the book fails to sell x amount of copies, etc). Breach of contract is something they can and will sue you for, on top of dropping you like a hot potato and never publishing anything for you again.