This article assumes you are using Topspin purchase buttons on the site where your GA code is installed.
Best Practice Walkthrough
Log into your GA account and navigate to your profile dashboard.
Select the "Traffic Sources" link in the left navigation pane.
Select the "All Traffic Sources" option in the sub-navigation.
Beneath the graph in the displayed report, you'll notice a set of tabs that should include "Site Usage" and "Ecommerce." If you do not see an Ecommerce tab, ensure you've met the prerequisites listed at the top of this article.
Click the Ecommerce tab.
You should now see a table that displays transactions, revenue, and conversion rates by referrer (see example screenshot below). This is powerful as it shows you the leading drivers of both traffic and revenue...i.e., you can tell what referrers drew a lot of attention AND what drew a lot of attention "and interest'' in terms of sales conversion.
GA gives you the visitor data out-of-the-box when you setup and install your tracking code. The sales data comes from the Topspin purchase buttons. They've been specially designed to integrate with GA's ecommerce logging framework to give you the "sales by referrer" view.