this business of a seller only leaving positive feedback AFTER the buyer has left positve for the seller IS A CROCK!!!   When the payment is made, the buyer has fulfilled his part of the bargain and the seller should leave feedback to reflect that. ...

It's about both parties doing what they're supposed to do and agreed to do. The buyer paying in reasonable time and the seller giving them what they bought in the listing. Sometimes the product arrives damaged in the post or something else, so it's just cautious by not leaving the buyer feedback because you don't know how the item has travelled until they leave their feedback.

eBay does handle disputes so I guess sellers should be more confident about leaving positive feedback right away.