EBay seem to give you a higher seller rating if you say you’ll post faster- which I’m sure will encourage people to over promise. And, well, things get in the way. I reckon you have to take into account that these are mostly not shops or professionals, sure they should post on time but with the best will in the world, it won’t always happen. Definitely not a sign of a problem at least but it is frustrating.
(I sell tons on ebay… When I miss a post date, I do tell people but weirdly the only delivery time complains I ever have, are when the item arrives within the expected window but they’re pissed off because they inexplicably thought “dispatch in 2 days” means “received in 2 days”. Nobody who gets their items late for whatever reason, has ever complained :?. Oh, and the people who pay late or have bad address details are always surprised that a 2 day delay in payment means a 2 day delay in receipt, too)