I don't think it is unreasonable to say you will charge for the flight then refund it if the guy buys the share.
Saying you will charge (even if you don't) soon sorts out the tyre kickers from the serious buyers. Serious buyer think it is totally reasonable, tyre kickers suddenly remember the time and start running for the door.
Having said that I've given lots of demo trips and I don't think any of the buyers were not genuine.