While from the legal standpoint, I'm sure you are correct as this is not MY field.
I however feel that when a seller receives MY payment, he/she is obligated to leave me feedback as I have completed my half of the bargain.
When the merchandise is received by me, I post feedback for him/her as then, they have completed their part of the bargain.

The reason that I feel people use this ploy is to insure that they have a vehicle for retaliation at their disposal should you leave negative feedback.

I recently made an e-bay purchase and the seller told me that he would send my merchandise when payment arrived.
I questioned, with close to 400 positive feedback comments with no neutrals or negatives, why I couldn't send payment once the merchandise arrived?
After all what is the purpose of the feedback forum if not to insure some form of trust between buyer and seller?
BTW the seller sent the merchandise. I had already sent payment but was trying to make a point.
Why is the burden of trust ALWAYS put on the buyer?