Price and specs are very important. So are a good website, fast shipping, clear communication about order status, security (CC data), and warranty/customer support. Newegg has historically had a very good combination of these, as have many other successful hardware sellers.

Also important are what we don't want: spam, deceptive webpages that try to trick users, spammy emails, an unclear or difficult website, survey spam, byzantine RMA policies, limited shipping choices (USPS parcel vs UPS ground is not good enough), customer service people who can't speak/type local languages clearly.