Detailed Seller Ratings (DSRs) or 'stars' that are left when you leave seller feedback are very important for sellers, does it matter whether you leave a 4 or a 5 for a good transaction ? It certainly does-even though you might think giving a 4 is good -it is not. Getting rating 4s can lead to significant seller penalties. For example on the US site the new 'Best Match' default search option means that sellers who have lowered DSRs due to getting too many ratings 4s can have their listings dropped down the page so for example, a listing finishing in 2 days can be below listings finishing in 6 days for exactly the same item.
The other thing to be aware of is that for all intents and purposes a neutral feedback affects the seller's feedback percentage in the same way as a negative-the seller's feedback percentage is lowered when buyers leave a neutral.
By all means if a seller's performance is not acceptable leave the appropriate DSR and feedback rating. But please before you do contact your seller with your concern, the vast majority of us want to give good customer service and resolve any issues you have