Based on your assumption, which may or may not be right, I think it would still matter. If you're going to market it on the internet, you want to use a name and extension that are memorable - say if you put the name on a banner or text ad, you want someone to see it and remember it. If you just want to use search engines, a good name with keywords for your business will help. Plus, if someone finds your name in a search engine and goes to it, and doesn't bookmark it, they will remember it was "travelbooks.com" easier than if your site is called "allkindsofgreattravelbookshere.ws", for example. Plus, online competition will keep increasing, so you'll want a name that stands out from the rest, suits your business, and is easy to remember and spell. Or, an easy to remember brandable term, like "twitter.com" or "plaxo.com". I don't think it's possible to keep website marketing totally online. You need to market in print, magazines, business cards, email addresses, etc, the domain will always need to be shown somewhere.