While I agree that a seller should not require that a buyer leave feedback first. I disagree that a seller should leave feedback once they are paid.

In the few times I have sold on Ebay I have always waited until one of the following has happened before leaving feedback for the buyer:

1. The buyer emailed me and told me the item arrived and everything was OK.

2. The buyer left positive feedback

3. So much time had passed that it was obvious the buyer wasn't going to contact me.

The reason is that if you leave feedback as soon as you are paid you have no ability to change it if the buyer turns into a nightmare later.