Price comparison aggregators like Google Merchant (former Google Base) shopzilla, shopping.com provide a unified interface for e-commerce websites / vendors - which can submit their product listing through a standardized product feed (example).
Visitors can then perform a comparison search in one place for all those products from different providers.
I'm looking for an alternative / an open-source engine that could be tweaked so it should work like those.
Alternatively, I wonder if one could rather use the Google Merchant framework and embed it in a website (like Google Custom Search)