The legality would be very much debated.
If you are making no money from operating an exchange, you would have a pretty good argument against being in the business, but then again if you are making no money why would you want to take the risk?
You have other issues to worry about like promoting gambling, gambling across state lines, etc. If you became a big deal the government could find ways to cause you problems most likely.
In any case, there are possibly ways to try to do this in a legal fashion. The upside would be if online sportsbetting becomes legal you would have a great database of players and be a leader when that happens. The downside is if you were at all successful, the DOJ would likely arrest you or at least challenge the legality of your website and who wants to spend millions in legal bills on a site that isn't taking in revenue?
I'm no lawyer but I just can't see this being successful and not seeing some kind of problem.