I can't jump on the responsibility of the seller BandWagon though. No matter which way I look at it, the seller can't determine critical things, in the buyers future path. What the buyer has done, and personality is like, can be seen/judged/evaluatied, pretty good... but, no matter what, it is the buyers responsibility to be responsible, period.
A private seller can at least ask to see the buyer's drivers license, and verify that it has a motorcycle endorsement.
If the buyer refuses to show their license, the seller has no obligation to sell the bike.
That's a very minimal standard, and it's easy to argue that an endorsement doesn't guarantee competence or common sense, but given the statistics that budman quoted, it will weed out the highest risk individuals.
To address the suggestion that a group like BARF should offer rider mentoring or training; that's a huge undertaking.
Consider the risk of some "mentored" newbie crashing and taking his mentors to court for damages. That would require at the very least, a large insurance policy for the organisation offering the service.
How do they select qualified trainers? Is there training for the trainers? Not just on "how to teach someone to ride", but on the legal responsibilities, etc. that might be involved.
Where do they do the training? Is it a good idea to offer a formal "mentoring" session on a public road, where any number of things might go wrong?
I could list other issues, but I'm late for work as it is...