Getting and keeping real estate listings up to date is a big pain in the ass. There's no standardized residential listing data, and while you might get RETS feeds or data dumps from the MLS nightly, it's still on you to ensure that all of the information is parsed correctly and displayed in accordance to the MLS'es standards (or risk being cut off).
Often, MLSes have terms in their contract that say that regardless of the source, that their information must be the canonical source for information in some cities -- which is a huge problem with dealing with REO'ed (Real Estate Owned) properties and the banks that want to sell them.
I know of a startup that eventually just resorted to scraping realtor.com (and I'm sure that they're still doing it to this day) instead of dealing with the various headaches of managing the contracts with the MLSes and RETS providers.
Often, MLSes have terms in their contract that say that regardless of the source, that their information must be the canonical source for information in some cities -- which is a huge problem with dealing with REO'ed (Real Estate Owned) properties and the banks that want to sell them.
I know of a startup that eventually just resorted to scraping realtor.com (and I'm sure that they're still doing it to this day) instead of dealing with the various headaches of managing the contracts with the MLSes and RETS providers.