I have a sore spot for sellers who don't leave feedback for you. I recently purchased several higher dollar items from various sellers, some of whom frequent Style Forum. I left positive feedback for them and politely asked them (multiple times) to consider doing the same for me. No response, no feedback. Next time, I won't leave feedback for them, or will consider leaving neutral feedback, as I consider this to be inferior service.
I do agree, however, that it's bad form not to leave feedback as a seller once it's left for you. I promptly leave feedback as a buyer when I receive the item and am satisfied with it. As a seller, I promptly leave feedback when it's left for me.
Please keep in mind that a lot of ebay sellers are like me - one person working very hard to complete all phases of a business by themself. During the summer I'm doing 70-90 transactions a month. Leaving feedback for each transaction as soon as the buyer does so is simply not feasible, not enough time in the day. I leave it in batches - at least 25-35 comments at once. So if you purchase something from me, please do not feel snubbed if a positive doesn't arrive right away, it will get there eventually, I promise..