I wonder if there is a way to modify/configure viart for using it as a price comparison website? Because it has mostly all the features in order to do that. For example it has:
*Product import feature from csv (it must be improved for XML import)
*Matching products by name or product code
*Merchant Module
So I thought that how can I use it as price comparison engine?

I cannot see how.
 
I presume you are in the UK?
 
This is a massive but highly competitive market place and the only way to be there is to subscribe to a research house that will update your products and prices daily. They will also supply the software.
 
You also need to ensure that you are FSA compliant.
 
Chrid