Pretty sure this won't really be a thing, at least in an official capacity. The entire point is that players set prices, based around supply and demand. If you live in an area with lots of horses, horses will be cheaper, because they are easier to find. It wouldn't make sense for someone to bring up a web page and say "well across the continent horses are being sold for 4x the price, therefore I'm going to sell you this horse for that same price".
Regardless, due to the nature of gaming and the internet, someone is going to do this, in an unofficial capacity. My hope is that it doesn't break the immersion of the game too much.