This is a mixed issue. FSBO owner can take time and expense for advertising, showing the home, etc that an agent will do for you. If you are buying and selling a lot of homes you should deal with one agent/broker and they may give you a deal on commissions due to the number of commissions you bring to them. There are people out there that won't buy from FSBO because they feel an agent will protect their interest.
There's always the option of doing FSBO for the first 2-3 months and if you don't sell then list with an agent.