Realtor.com and the MLS are two very separate sites. Realtor.com is operated by MOVE, Inc. Realtor.com downloads property data from the hundreds of local MLS’s and makes that property data available to the public.
Realtor.com is not a “For Sale By Owner” website, so seller’s contact information is not available or permitted to be posted. It is acceptable to list the seller as the property contact on the local MLS because it is only accessible to Realtors.
The MLS advantage is that every single broker and agent participating in the MLS has an incentive to sell your home, effectively putting every agent in the area to work for you! The first place a buyer’s agent looks is always the MLS, so there’s simply no better way to gain instant and widespread exposure for your home.