I allowed a water test yesterday, and didn't ask for money.
The buyers had, however, demonstrated clearly that they were interested, by having a mechanical check and then flying down from Rockhampton. This made it easy for me to assess whether they were serious, but the point is clear - take the time to establish they are serious, then take the chance, IMO, if you think they are.
By the way, they bought Bubi, and she's on her way to Rocky next week. Keep an eye out for her, folks!
Cheers,
Tim