Your friend's well thought-out steps to protect himself were all great ideas and worth doing. However, do keep in mind that:
1. You can't waiver away negligence. E.g., if it could be proven that you knew your bike had faulty brakes, but you said nothing, and their failure caused the interested party damage, your signed waiver is useless.
2. The psychology behind holding one's valuables is excellent, but it's questionable as to what you can legally do with the interested party's stuff. Courts tend to rule against holding property as ransom, not to mention, the only item whose value would cover the loss of your bike - the guy's car - wouldn't help you unless he's willing to sign its title over to you.
3. The psychology behind holding one's credit card may be effective, but you can't actually do anything with that card.
4. A follow-up test drive by the seller is a great idea unless the interested party denies having caused any difference(s) in the bike's condition or operation. Might be best to video the seller proof-riding the bike immediately before the interested party steps aboard to hopefully be able to demonstrate before and after comparisons.
This really is a tough call. If I were selling, I'd inform the interested party that the dealers in our area require payment in full before allowing a test ride on recreational vehicles such as ATVs and motorcycles. Then I'd probably have them sign an agreement that they'll owe me my full asking price if they damage my bike in any way (proven, with luck, by aforementioned video). But then how do you keep the guy from just taking off, and then if you find him, who wants to drag him to court?
The answer now seems obvious .... we should NEVER SELL our beloved Spyders!!