You can add Buy Now buttons to any rich text editor. To do this you will need a button (free buttons available to download) to add to your site saved as an image and a price list ( click to add an example shop price plan to your Price Plans Manager). Setting up a Shop price plan will provide you with one-click links to any products or for products within a normal (print and product) price plan you can create 1-click links to add a products directly to the shopping cart.
For the purpose of this tutorial we will assume you have created a shop price plan and have a Buy Now button sat on your desktop at the size and style you want to display on your web site.
UPDATE - The new "Simple Button" makes this easier, faster and better. See the help article here: How to add a Call-to-Action button (CTA) to your web pages