Stores often try and sell things as cheap as possible, so being able to beat this is going to be difficult, especially against the likes of amazon. There is a reason they set those prices!

There is a company in Australia called bing lee (http://www.binglee.com.au) that allows you to do this. You can either barter in store or via their chat on their website.

I don't see this idea being successful easily. Driving people to your site and making them interact with you would be difficult. I personally couldn't bother with such a site, too much effort. I'd rather see whatever is cheapest on a few different sites and go from there.