I'm not sure I understand how viable a protection this is...just seems it will shut out newer people with certificates that would like to be able to sell them.
Why not just have suggestions for people to follow before going through with a purchase. Ensure you have a contact number, even ask for government ID, something. Force payment via PayPal with buyer protection? Use an escrow service...
Can't anyone build up 25 posts if they just want to scam someone?