Been seeing a lot of whitepapers of new exchanges that claim they can aggregate order book from different exchanges to allow for best rate and coin options for investor. While the idea sounds great, I wonder how this will actually work?

Given most of all the exchanges are centralized, if a user from Exchange ABC would like to buy Bitcoin from Exchange XYZ, how can trade be handle seamlessly? Also, once trade is across different exchanges, how does one go about ensuring proper and valid completion of a trade?

Would love to hear any comments and thoughts