So this is a common question, and I have a two part answer for you.

Some realtors may avoid flat fee listings if it is a For Sale By Owner (FSBO).

The realtor thinks they have to do double the work and in some instances they are not offering a cooperating broker fee so they will have to negotiate their fee with the Home seller.

In some cases this is true.

There are a lot of deadlines and requirements that for sale by owners may not know about so the real estate agent will have to inform them and teach them along the way, and a lot of real estate agents won’t have the patience to do that.

Selling a home is a full-time job and agents will need to be able to get a hold of the other agent or for sale by owner (FSBO) and if they are not able to meet the deadlines and the deal may fall apart and all the work they put into it will be for nothing.

On the other hand if you use a flat fee or a value broker to list your property you have an agent representing you that is available day and night and weekends. They will not have an issue showing your property or recommending your property. 

Back 30 years ago real estate agents would have to know what properties are for sale in their neighborhood.

Now consumers are more educated because of the Internet they have Zillow, Trulia, to look up property values and what properties are for sale in their neighborhood.

The new modern way is the consumer or client or Homebuyer tells the Real estate agent the homes they want to look at due to the virtual tours pictures and the description online.

Homebuyers look at homes online for 60 days before they ever even contact a real estate agent. So imagine if a traditional real estate agent will not show their client a property that they want to look at because they are a flat fee real estate company.

The client will want to know how does that affect them from buying the home.

So in summary most all agents will work with a Flat Fee Realtor as long as they offer a Cooperating Broker's Fee.