This is one of the disadvantages of selling by owner. You need to know that real estate agents pay monthly advertising fees to websites like Zillow, Trulia, etc. In return, these websites allow agents to have ad space with their contact information so buyers can call them. There is a default checkmark next to the agents and buyers need to uncheck it and put a checkmark next to owner. Very few buyers do this and realtors know it.
Once the buyer calls the agent, that agent is quick to tell YOUR buyer that your home is not for sale, is sold or some other lame excuse. Realtors don't bother to check or verify the status of your home. Instead it's much easier for that agent to tell YOUR buyer the following: "I'm sorry but that home is not listed for sale, however, I do have other homes to show you. Can I schedule an appointment to meet with you and show you the homes I have available?"
Did you notice the agent doesn't mention that all they check is the MLS system - not Zillow or Trulia. The MLS system GUARANTEES that the agent will be paid a commission because there is a contract in place. As a For Sale By Owner, the agent would have to negotiate with you to receive a commission. Most agents would rather lie to the buyer (as above) instead of negotiating a commission with you.
You can test the above yourself. Pretend you're a buyer (from outside the neighborhood) and ask the agent about your house. See what they tell you. You do have an alternative. You need to research "Flat Fee broker" (Google it) and this is an alternative to For Sale By Owner. You save yourself about 3% in the commission fee.