If you have listed on the MLS with an agent, then you have listed this way.
We list on the exact same local MLS as any other agent. Listing on the MLS for a flat fee is the same thing as listing with a 6% REALTOR®. The only commission posted on the MLS, is the commission you are paying the buyer agent. All listings are in the same format, on the MLS. When you look at websites like realtor.com, notice all listings are posted the exact same way.
You can sell to someone not represented by an agent and NOT pay a buyer agent commission, however if an agent brings a buyer, that agent will need to be paid a commission.