Feedback is voluntary. Feedback should not be expected on either side of a transaction.
Since buyers can only receive positive feedback, as a guage of reliability as a buyer, its useless. Unless a buyer is planning to sell and wants a few feedback under their belts to get going...its really not necessary at all.
My best advice....focus on the items and less on the feedback overal. It will make your stay on ebay a more pleasant one.
The reason why ebay reminds you to leave feedback for a seller is because there are potentially enormous impacts of feedback. The number of negatives and neutrals as well as the detailed seller ratings determine how many items a seller can list and even how much they are charged for listing items for sale.
You might find that you'll still receive feedback on those items that don't have it yet. Some sellers leave feedback in bulk at various times of the month.
BTW, you may be unaware but you've wandered onto the ebay Canada website. Just in case you were wondering why things looked a little different.