For most, it has to be Flipkart.
Their website is simple and extremely easy and clutter free to use. They have a robust customer service and timely delivery. Also, an extensive range of product ensures that people keep coming back.
Infibeam is good as well but, personally, I've always preferred Flipkart.
eBay is known far an wide but has a lot of bogus products that accompany legitimate ones. It's just not easy to trust it and knowing the Indian consumer, it's easy to assume that they'd prefer something safer.
The option of Cash-On-Delivery also plays a major role since the middle class is yet to submit to the addiction of plastic money!
www.D880.com is another trustworthy portal that caters to health product requirements of the increasingly health conscious urban youth.