Actually, I rarely get the free shipping deal... because most of the time, I do not buy directly from Amazon, but from one of their bazillion vendors.
Amazon has a feedback system, but more important, they offer buyer protection for things ordered from their vendors. I have NEVER had a problem with an outside vendor there. I did have a problem once where I was shipped 2 of the same part, but missing 1 of another. I had the replacement within 2 days. That's a much easier way to do it (have someone pick it up from your door and bring you a new one) than standing in a store line with a cart and returns, the gas to get to the store and your valuable time wasted doing it?
I know, I sound like an iPod groupie for Amazon, but I see NO reason to look at another online shopping company ever again. About the only way I'd even look at Goog or Yhoo's shopping sites is if Amazon went out of business. That's how much I like Amazon.com. I would guess most people who have ordered more than a few things from them feel the same way.
Oh... and gift giving? Find an item, click, "Buy It Now", enter their address and what you want the card to say, and you are done... and they'll get it in 2 days. I sent Mother's Day Flowers and chocolates 2 days before via Amazon last year. Christmas shopping, Birthdays, Weddings and Funerals, I've never been able to spend my time this efficiently ever before. I can solve a social situation in 5 minutes, with 1 eye on the market.
Yes, there are other shopping sites, but Amazon and Ebay are pretty entrenched. I am happy. I see no reason to be Goog or Yhoo's guinea pig while they learn how to be like Amazon, because I can just go to Amazon. Plus, Amazon have this awful feature that records all the types of things you like, and shove it in your face next time you appear on the site, so you can spend still more $$.
Lets say you are an amateur photographer... you look at some photography books. Next time you open the site, you will see nothing but photography books along the same lines you looked at, new cameras, lenses, etc. Or stock market books. Or the latest DVD's... you can sign up now for the movie you just watched at the theater and preorder the DVD so you have it at your door the second it comes out. Video games, yesh, you name it.
Why would anyone need another shopping site? If you are used to using Google, what would prompt you to start searching on MSN? You wouldn't. If all your music is on an iPod, why would you switch to Zune (unless someone showed you the difference)?
The lead time Amazon and Ebay have had established loyalty, and people are creatures of habit. Unless they start doing something stupid to their customers (ie, Home Depot cutting back on sales help), people will stick with them, and they will tell their friends about them, and not another service.
Is goodwill a commodity? You bet. And they've been at this for years now, they have worked out most of the bugs, and the service is fantastic. Who cares what new shopping service crops up? What can they offer that beats perfect?