How could Google define "low quality experience merchants"?

[BrianSaxon]

Matt Cutts mentioned at SXSW that Google wants to take into consideration the quality of the experience ecommerce merchants provide and work this into how they rank in SERPs.  Here's what he said if you missed it:

"We have a potential launch later this year, maybe a little bit sooner, looking at the quality of merchants and whether we can do a better job on that, because we don’t want low quality experience merchants to be ranking in the search results.”

My question; how exactly could Google decide if a merchant provides a low and high quality experience?  I would image it would be very easy for Google to decide this with merchants in their Trusted Store program. I wonder what other data sets Google could realistically rely upon to make such a judgment.  Any ideas or thoughts are appreciated.

[GKLA]

I would agree that the Google Trusted store would be a good place for them to start, very convenient.

They could also use ratings from other review sites, like Yelp, etc.  and possibly even social signal via Google Plus, Twitter, etc.

[kadesmith]

I can see them doing something with longevity tracking and repeat customers.  They could possibly track to see if people are return shoppers to a site.

And then there is the obvious reviews of a store from Google Wallet users when you are a Google Wallet store.

I think there are other things that they could do just in the design of a site.  They could possibly figure out what design elements make an experience easier on the shopper.