There are lots of successful aggregators: NerdWallet and Credit Karma for credit cards and financial products, all the different car shopping sites, also Trulia/Zillow for real estate.

All of those companies have a clear value proposition for buyers (one place where you can get information instead of lots of scattered websites that might or might not be helpful). They also tend to have strategically strong customer acquisition channels that give them some degree of leverage with sellers (i.e. they can say, do what we want or else we'll stop passing buyers through to you).