As a buyer I admit sometimes I forget to notify the seller, it's not intentional, I just get busy. Also if the seller didn't send me a notice or any email saying it was shipped then I don't have his address in my email for an easy reply. Those are the ones I usually forget to notify because now that I am a very casual ebay buyer/seller I don't always log into the ebay account and go to the "my ebay".
As a seller, when I ship I only use a carrier that provides true tracking. I can log on and see that it was delivered and when it was delivered. I figure when I see it delivered and if I don't hear from the buyer within a few days then everything must be good. Selling on Ebay for over 12 years I've only had one or two small issues selling (other than Ebay fees). Like many I once sold heaps on ebay, now I sell very little. Last year I sold a total of 5 items, I once was selling over 5 items a day.
