You know what annoys the crap out of me? Buyers who don't communicate to the seller.
I sell stuff on eBay, which makes me fairly anxious because the post service in my country sucks. It's relatively common for items to be "lost" (read: "stolen"). Furthermore tracked parcels are relatively expensive, so if I'm selling cheap items (books, comics, etc) many people will only buy my stuff if I offer a cheaper shipping option, which means no tracking, which means no protection from chargeback scams.
Despite that I keep selling on eBay because, frankly, I need the money. So, when I ship an item, I look forward to confirmation that it arrived safely. Positive feedback brings a huge relief not just because it improves my profile, but because it means that I actually get to keep the money.
Sometimes I get buyers who go total silent. They pay the item, then disappear into the ether. It drives me fucking crazy. JFC how hard is it to reply to a message or give feedback to confirm that the transaction went well? I keep fretting that they didn't get the item >_>