I'm building a computer for the first time for a friend. I usually buy stuff from the local stores but I save almost $600 by buying stuff online. I LOVE using pricewatch.com! Almost every part I choose from pricewatch, comes from a different online retailer. How reputable are these sites? How do I know that they won't take my credit card number and run?

I'm just a person who really worries a lot.
Thanks a lot.