There seem to be no ebay guidelines as to who should leave feedback first, the buyer or the seller. In fact, ebay's policy just states that leaving feedback is optional. In effect, leaving feedback is a bit like saying 'thank you'. It is just a polite response, and whilst a failure to thank someone would be regarded as very rude by most people, few of us would do anything about it, other than ignore the rudeness.
My local shopkeeper is a bit of a character, but his manners are impeccable. When I buy a loaf of bread (or anything else) in his shop, he will ring the sale up on his till, then request the money. When I pay him he will give me a very emphatic 'thank you'. If there is any change due, he will hand it to me, after which he will hand me the bagged goods, thanking me again in his usual theatrical manner: 'Thank you, sir! You are a gentleman and a scholar!'
After that, I will take the goods, thank the shopkeeper and leave.
Granted that the shopkeeper's 'gentleman and scholar' bit gets a bit predictable after a while, he nevertheless does follow the age-old convention established by people. Once the cash is handed over, the customer has fulfilled his part of the transaction, and one would expect the shopkeeper to say 'thank you' at this point. By then handing the goods to the customer, the shopkeeper fulfils his part of the transaction, at which time it is the customer's turn to say 'thank you'.
Buying and selling on ebay is no different. By paying for the goods, the buyer fulfils his part, and should be thanked by the seller in the form of feedback. When the customer receives the goods, the seller's part is complete and now it is the customer's turn to leave feedback. Even if the goods are unsatisfactory and have to be returned, the customer's part ends with paying for the goods. Returning goods or accepting the refund do not affect any aspect of the original payment.
Now, as I said at the beginning of this guide, most people would respond to rudeness by simply ignoring it. And this is exactly what I do when a seller is so rude as to not leave feedback. Sellers who do not leave feedback should not expect to receive any feedback from me.
Occasionally, I receive an e-mail from sellers (who did not leave feedback) reminding me that I have not left any feedback for them. I can never be sure whether they are trying to be funny, cheeky or what, but my response is always the same. I will simply remind them that they have not left me any feedback either!