I've been an Adwords consultant for 3 years now. I've worked for various different agencies working with major e-tailors, ironically I also worked for an agency that managed eBays paid search campaign too. Recently I started freelancing under my own name.
Before that I made a living through an ebay store, but I soon got tired of it. Aggressive competition drove my margins down. Plus I love marketing, I didn't quite like the physical aspect of packing goods, dealing with suppliers, sourcing goods and constantly having to re-negotiate costs as suppliers always bumped up the unit price all the time.
It's only when I started a career as an Adwords consultant that I realised that many e-tailers on eBay and the like were using eBay primarily as a lead generation tool. They'd sell cut price goods on eBay in order to build a list of clients and they would remarket to those buyers through email and push them on their ecommerce stores.
In addition to this they would also buy traffic through Adwords in order to convert to sales. The reason being it's possible to upsell and cross sell more products to the same buyer on an ecommerce site. Also eBay has a knack for being "cheap" so prices need to be very competitive. On your own site it's possible to charge fairer prices meaning more profit for you.
I've worked with a number of different Ecommerce retailers helping them increase their profits by cutting down advertising costs through highly selective keywords ad bids which cut down on untargeted ad placements. I focus on CPA (cost per lead) primarily. By getting the CPA down as low as possible.
If you would like to find out more about my work, visit my website: www.karimium.com