I do leave feedback on all positive purchases, just as a matter of courtesy. If something minor was wrong, I contact the seller first. Almost always, the seller makes it good or substitutes the product. When I do sell, I encourage positive feedback, and request contact if something does not please the buyer. So far I have a 100% positive feedback rating, so I must be doing something right! I have had a very minor percentage of buyers who do not leave any feedback. I have to assume that everything was OK, and they just didn't take the time for feedback. I do not pursue those buyers for feedback.
It was far more valuable in the "early" days prior to photos and helps get new sellers off the ground. After I passed 1000 as a casual seller of all sorts of material, mostly hobby, it became moot for me to constantly request feedback, as the seller, etc. as it seems more of a annoy post. If they wish to Feedback, great. I always provide feedback to buyers. Just my 2¢
I sell just to sell, but I can understand for those who want to build up a reputation, then feedback can be considered more important.
Some people just don't leave feedback. It happens and I wouldn't worry too much about it. I'd caution against messaging people to ask for feedback if they haven't left any at all. I've seen a few buyers leave feedback that was something like this: "You want feedback, well here is a negative" Now that is rare, but there are some characters out there. With those cases, I'm happy if they don't say anything at all.
I'm one that lets it pile up for a few months, then submit it all at once. I do get to it eventually. One exception is free shipping, I might give positive feedback on that quicker. I've only had one coin where I felt the item wasn't exactly as described, but really, it was that one aspect of the item WASN'T described, not that it was described falsely. If a coin has an obvious cleaning, I prefer for the seller to state that and most of the time I'm expecting the seller to know about the coin they are selling adequately enough to make those disclosures, although I realize for some sellers, providing detailed descriptions beyond the photo of EVERY coin they offer is asking a bit much. As a side note, if any of you on here sold an AU details 1875-S twenty cent piece today, I'm your buyer! @ddddd warning! I'll be posting it on your 7070 thread soon hopefully!