You should also scrutinize how a site looks. If it seems as if the site hasn't been updated in days, weeks, or longer, that can be a sign that the company isn't paying attention. Just as you wouldn't buy something from a retail store that looked shabby, exercise the same caution with an online vendor.
Layout and design are also important. If the vendor spends the time and money to build a great-looking site with good search capabilities and easy navigation, you can be sure it is serious. Good design is a sign of good organization and clear purpose, so browse any merchant site you're considering buying from as a way to find out about the people behind it.
Online commerce is fast becoming the third mass-market revolution, after magazines and television. Catch the wave, because it's clear you can get better selection, faster service, and better prices shopping on the Web than anywhere else on the planet.