Potentially it would be a lot of work to keep up to but as you said previously that if it can prevent at least one person from getting ripped off then it is a success, this aswell as implementing a time period before a person can sell something. Maybe SAU needs more mods/staff.
I reckon the people that have already proven themselves as honest, reliable traders will not have the third party request by the customer as much as a new member selling stuff. So the load may not be as much as we think.
So as long as the option is always available and all sellers agree to co-operate with it then it should work.
10% commision for SAU seems a bit too high. 3-5% maybe??