As the local realtor, you need to keep abreast of what's being offered. Don't lie and tell them you have a buyer unless you really do, they'll have already heard that line. It's just another viewing, however you bring great value in that your a goto person when it comes to realestate. During the showing offer them something of value, a weekly area sales report, updates on competitive listings.
When a FSBO asks if I have a buyer, I say 'none that are currently interested in non MLS listings'. This always opens things up for discussion. A real quick overview on current sales trends, competition, and your services will have them saying yes to a listing, or at least yes to regular contact where you provide a message of value(area sales report) and a repeat request for their business.
FSBOs are a specific prospect segment. Be prepared by carrying your materials with you. And as soon as you see that sign, stop and knock.