First off Ebay is not new and definitely running out of room to grow. Amazon is just as old as ebay and has grown a lot, but maybe it can do a good job adding additional vendors. As long as they are reputable - amazon can still grow. priceline and netflix are new. Most people new priceline during the dot com bubble but they have done a good job in growing. But they have plenty of competition in the online travel sector. Netflix did a great job crushing blockbuster. But they have competition form video on demand, itunes and hulu.
Any profitable company will have competition.
Apple has many competitors, but they are also in many spaces making lots of money. What I would like to see them do is get in the mainframe business and knock on the doors of ibm and dell.