I just pass judgement on the individual I'm dealing with. You can usually spot a cunt on the first communication with them and you need to trust your gut instinct.
If buying then I want to see them at their address and see some i.d. IIRC I've been allowed to test ride all but one of the bikes I've bought and the one I wasn't was in as new condition anyway so I wasn't worried about it.
I've let people test ride the ones I've sold but I've screened out a few potential buyers that didn't sound like decent people.
I haven't been stung yet. I don't buy/sell GSXR's though so I screen out most of the low-class criminal types that way.