I've mixed opinions on that. For example, there are products like Google Shopping or Google Travel etc. They show up bunch of stores where you can buy a flight ticket or a particular product from.

However, in the end most people just end up buying it from top 2-3 marketplaces anyway, not because its cheaper, but also because they can trust it, it arrives in 2 days, and you can easily return it if you don't want. So if I'm going to most likely buy something from Amazon anyway, why not just use Amazon web page. They've had one click buy button for ages.