Like User-10190343953443518142 mentioned, you will likely be dealing with a small company as there is not a national brand that belongs to all the MLSs out there.
The important thing to be concerned about are the details in the listing agreement that they have you sign – a listing agreement is required by law. Here are the big things to look for in a flat fee MLS listing:
1. The listing MUST be on the local MLS.
There are hundreds of MLSs in the United States. Sellers want to be on the MLS that covers the area where their home is located, so they can offer a commission to LOCAL buyer agents.