Develop your niche from your online store, and bring that to virtually every area of the business.
Create eBay, uBid, Sell At Once and other places to be an extension of your store and therefore a place where you can without reserve open up your niche market and guarantee 100% satisfaction.
If you can consistently satisfy your customer base, even if you aren't the biggest or the smartest or anything else.....you will have the most success, and your niche business will be at your command and able to be tapped into all over the world.
That's right don't listen to the experts.
Don't ever sell one thing over and over again.
Develop and secure the niche markets tailored to you, and then sell a different thing all the time and watch it go bigger and bigger.