Google Checkout has introduced a new, embeddable gadget to allow you to sell your products or services on any website in three simple steps.
First you need a Google Checkout Seller Account. Then you can use a Google Docs template spreadsheet to list the products you want to sell.
You can then copy this template file over to your Google Docs account where you'll see a basic spreadsheet that includes columns for title, content, price, quantity, shipping, image_link, option, and option_name.
Once you've populated your spreadsheet you can publish this as a web page and then choose an embed location - either a regular HTML website or one Google's properties (Google Sites, Blogger, or iGoogle).
Just choose the size of your gadget, make a couple of changes to the code (e.g. pasting in the URL to your own Google Docs spreadsheet) and you're done.
While PayPal offers many more options for using its buttons on your website and configuring the details for your products, Google Checkout offers a simple shopping cart option that's worth considering.
Source: Sarah Perez, "Lookout Paypal! Google Checkout's New Gadget is Incredibly Simple," ReadWriteWeb, July 31, 2009