An article has been brought to my attention - tips for protecting yourself while shopping online. I found the article to have a number of good points, many of which could be elaborated.
I agree that the most important thing to notice is the URL bar and whether or not it says "https". Some commenters made additions to the shopping security checklist. For example, don't enter your credit card details from a public computer (ie. internet cafe). To add my own points to the article, don't send your credit card details through email - email is notoriously unsafe for this kind of information. Make sure you enter these details on a trusted, secure site.
Online Shopping Protection