Also, the variance in shipping costs and sales tax/vat are additional way to help improve comparison accuracy, but getting this right is a whole lot of work, and worse, keeping it up to date is painful.

Another consideration would be how various browsers are being faked. If you're spoofing by using a modified user-agent, then the spoof is detectable through javascript. Getting past this could be done either through virtualization or creating a modified client/browser to better emulate the target browser.

It's a great project, and I'm looking forward to reading though your code.