As a newbie retailer, I can't really speak as to why some who have been around awhile aren't doing it. I know why I'm not doing it (yet), and that's the time it takes to do it. I'm already working 60 hours a week in my storefront, and I have a family. I don't even have significant help in-store. (I have one employee who works about 5-10 hours a week) Having spent time selling other items on ebay in the past, I know how much time it takes just to deal with the listings and payment management, and shipping items out in a timely manner. I just don't have that kind of time.
I agree that it's both a good way to rid yourself of dead inventory, and a way to generate some quick cash-flow.
Will I sell online in the future? It's in my business plan, but not for a while.
"While we were there, we took some pictures of the native girls, but they're not developed yet, so we're going back next spring"-Groucho Marx