Placing action buttons in the right position is something that could improve usability and the whole user experience at your site. Action buttons are used when user input is required. For example a "vote" or "add to cart" button (see more on action buttons).

"Placing a checkout button on a visible location is very important and will prevent users from flying away…"

The checkout button is an action one, it’s used at online shop websites to start the checkout process that finishes a purchase. Placing a checkout button on a visible location is very important and will prevent users from flying away from the website just before buying the products. Once again, this is a clear example of how usability can have an important impact on sales.

Best Buy, the online retailer for electronics, places the checkout button in a position that could be out of the sight of the user if she has a small screen or if the screen resolution is too big. In this example, the user has to scroll down to see the checkout button (below the red line).

Amazon has a smart design, they place the Proceed to Checkout button not only at the bottom of the page but also in a very top position. This way, there is no chance not to see the checkout button.