I should have mentioned it, but I think "real-time" would be overkill
(i.e. live forex quotes), but generally recent currency exchange rates
are easy to get.
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.