From your point of view, give us the best arrangement for these factors in sale procedure customer, employee and the supporter (agents)?

The business (Service or Product) always needs a proper connection between its departments to ensure good service has been provided to the clients (As the client is a main income for the profitable organization).
In my opinion and from my small experience: To have a peace of mind and avoid all these questions and procedures !! By having a shared-clear-strategy and policy between all departments, with a good trained staff and having a good Facility (That is the major thing and would help SO MUCH).
Next just to monitor the performance to ensure that everything is going smoothly (even once there is an issue, it will be easy to control, discover and get it fixed as per the company policy ASAP).
But the more important question that we should ask ourselves is: Where and How to secure above needed elements easly at one organization !! Here is the big challenge which has to come by the efforts of all departments together to choose the best for it's organization, then that would end up with good result.
Regards,