If you are familiar with the stockmarket or investing, then you know it's a good idea to have a diversified portfolio. In plain English, you don't put all your eggs in one basket. Internet marketing is no different. To earn the most profits and achieve maximum success, you need to offer a family of products. That's true, you still need a focused approach, but that doesn't mean you can't diversify your offerings.
Chances are theywouldn't last long.
You spend your marketing budget advertising these great new CD players. You point them to your CD site where you earn a commission on their purchase. You point them to your next site where they can buy batteries, headphones, carrying case, etc. Of course, if you happen to own an electronics store or a music store, you'd get ALL the profits from those purchases, but that probably isn't thecase with most people. Besides, if you owned the store you'd also have all the headaches associated with it, i.e, stocking, shipping, staffing, etc.