Contrary to popular belief
Having the cash in your hand does not entitle you to keep all, or part of it in any circumstances,
If your potential buyer asks for his / her money back, even if they have crashed your bike, and you refuse to hand it over, you are breaking the law and can be prosecuted.
Your test pilot needs to have adequate insurance cover, but bear in mind that many policies do not cover you to ride any bike by default.
Those that do, tend to only offer third party liability.
If your test pilot is not covered to ride another bike under the terms of their policy, and they get caught, you are just as culpable as them and will be prosecuted.
Make sure they have fully comp cover to ride another bike.
That's the only way you are protected against damage.
Yep, it's a mine field.