The biggest reason that sellers must give feedback first? 1) They are making the transaction to make a profit. They are beholden to the customer to deliver once the money changes hands. 2) I've never received one item on ebay without FIRST paying for it. Has the seller fulfilled his total commitment once he pays in full? ABSOLUTELY, YES. He should receive positive feedback immediately. Might he subsequently leave negative? Yes, this is the RISK the seller takes to make a profit.

The PROBLEM is the seller who won't leave positive feedback until he sees it from the buyer. The reality of business is RISK for PROFIT. Nothing wrong with that. But the seller must be willing to take that risk.