As harley said, eBays' postage delivery estimates are just that. Estimates. They are not promises or guarantees of the delivery date.

You say in your 1st line in the OP, and I quote "ebay sellers who deliver items a lot longer after the expected delievery date."  Sorry, but sellers do not deliver the items, they take them to the post office & Aust Post delivers them. There's a big difference. 

I live out in the bush, 300kms from Melbourne, & once had an item take 8 days to arrive from there. When I received the item the date stamp on the parcel showed the seller posted it the day after I paid. BUT it took Aust Post 6 days to deliver it to me, where it should have only taken 2 days. I gave that seller 5 stars for postage time because that's what they did, posted quickly. It's not the sellers fault that AP took their time getting it to me.

And that's one of the problems with that star rating. Too many buyers give low stars or negs & neutrals for that because of the time AP takes to deliver, instead of it's original intention of how long the seller takes to post.