Generally speaking the biggest competitor to any innovate website is the status quo and apathy. Every startup I have ever been in had a harder fight against people that were reluctant to move to better technology than against our direct competitors.
Presumably, realtor MLS isn't so much as a "competitor" to Zill/ulia so much as the dominating incumbent.
88% --- eighty eight percent --- of home sales (presumably: owner-occupied single-family residence sales) were done using licensed real estate agents in 2013. A staggering amount of money is funneled through that profession, which as I understand it exercises guild-like control over the MLS databases.
Generally speaking the biggest competitor to any innovate website is the status quo and apathy. Every startup I have ever been in had a harder fight against people that were reluctant to move to better technology than against our direct competitors.