For an auction site like eBay, more bidders means more listings, which attracts more bidders and yet more listings and so on. I have to believe that eBay has cared more about adding customers than ensuring reliability in its systems. If that assertion isn't true, then eBay has been incompetent. In their defense, eBay and the other mass-consumer Web companies are operating in a new environment. The Internet is a marvelous network of networks, connecting millions of computers worldwide in a way that defies systemwide failure.
Some computers work reliably when subjected to massive stress.
Some companies have backup systems that go on instantly when primary systems break down. Some of the top Internet commerce companies are subjecting systems to almost unprecedented levels of stress. A reason is not the same thing as an excuse. But the same thing applies to customers who expect something for nothing. They're the ones who love the idea of buying and selling stocks for an incredibly low price compared with an old-line brokerage.