If you are shopping online, how do you determine whether the merchant is trustworthy?

If you are shopping online, how do you determine whether the merchant is trustworthy? Do you use a different method to determine trustworthiness when you walk into a store and need to use a credit card for a purchase? Who is better positioned to regulate online transactions and handle irregularities or complaints - consumer advocacy groups or the government? You might need to do some research on the purposes and functions of consumer advocacy groups AND the method of filing a consumer complaint with a consumer protection office (government agency). references