Hello,
I had suggested a custom solution based on CodeIgnitor or Symfony2. The best part of a custom solution is, anything and everything is possible as it can be tailormade to suit your needs. But as understandable, the cost and turnaround time is higher as compared to an off the shelf solution.
I have seen the Etsy clone script from Agriya and I see that it has most of the the features you have requested, but not all. For instance,it does not have the option of choosing cutom product templates. Although it has an option of multi-store, it does not allow you to customize the store template like eBay. Also, the etsy clone script does not seem to have an ad management platform. But on the brighter side, it does have most of teh crucial features such as adaptive payments, internal messaging etc.
So even if you use an off the shelf solution (agriya or anything else), we will still need to custom code a lot of things. However it would be cheaper than scratch development.
To give you an idea....
For a complete custom development of all features and modules, the cost would be around $7500 to $ 8000at a timeline of 75 to 85 business days with testing. However if you use an off the shelf script and we add in the missing modules or customize the existing modules, the cost would be around $3500 at a timeline of 40 to 45 business days with testing.
I am agreed with your this point"Provide mockup of site before job is awarded"
"OfCourse the Choice is upto you"
Thanks,
Prasun