Let me preface my comment by first saying that leaving feedback is NOT mandatory it is a courtesy.

Let's reduce this to the lowest common denominator.

YOU sell me an item. Ebay requires me to contact you within 3 days. I contact you within 3 hours.
I tell you I will pay by postal M.O.
I send out the M.O. the following day. I e-mail you and say that payment is on the way.
You receive the payment 3 days later.
WHAT have I NOT done to complete my part of the bargin? Why would you not post feedback?

The only answers I can see

A] You want to insure I leave you feedback

B] You are not a reputable seller and are afraid I will leave you negative feedback

C] You are concernwd about me being happy with my purchase

BULLCRAP