Recently I've been considering whether Ebay or Amazon are actually worth it.  For the purposes of analysis, I worked a quick way of measuring the percentage of customers that come back and buy direct from me after having found us on Ebay or Amazon.

I wrote up a specific tutorial for Netsuite users on how to do this:

http://www.saynototheoffice.com/calculate-amazon-ebay-returning-customer-rate/

My results were not good.  Just 1.4%.  I wrote up another article discussing whether it was actually worth continuing selling on these channels:

http://www.saynototheoffice.com/amazon-ebay-selling-worth-it/

In short, they charge high fees, are a pain to administrate and don't seem to send many repeat customers, nor customers who Google you directly after having seen you.