Detailed Seller Ratings, are one of eBay's 'helfpul' policies.
Look at the descriptions for each of the stars, and you'd be forgiven for thinking that 3 stars indicates the sort of service you would expect. 4 stars is impressive, and 5 stars is exceptional. This is what the descriptions say, and how 'English' would, in most of the known universe be interpreted.
Not to eBay though. eBay has its very own dictionary and logic that is a mystery to everyone else.
In order to avoid being penalised by eBay, sellers need to receive an average of 4.6 stars across the board. So anything less than 5 stars actually means 'not acceptable'.
eBay will then take those 5 possible scores, and apply some mathematics, known only to eBay, and award the seller one of four possible ratings. - the top is "good" and the other three are different levels of "not good.". A seller receiving 4 stars (Acceptable) across the board runs the risk of being kicked off eBay completely for being unacceptable.
There is no 'very good'. In order to give a seller the top rating of 'good', buyers, en masse need to give a score of 'very good'.
Oh and as far as I can tell, such is the level of eBay's statistical expertise, that if you have received really bad service, then awarding a seller one star is substantially worse than awarding them no stars. Awarding no stars at all seems to be equivalent to 4.6., whereas 1 star will cancel out ten buyers leaving 5 stars.
I don't know if this guide complies with eBays policy on guides. The link to policy opens a guide to writing guides that doesn't have a link to their policy on guides. - But there is no profantiy, and I don't think anything here is inappropriate.