how to give stars to the seller?

[luminites]

Go to My eBay > Purchase History and click on Leave Feedback.

Or you can leave feedback (and stars) here: http://feedback.ebay.ca/ws/eBayISAPI.dll?LeaveFeedback2

Note that 3 stars for "Item as Described" or "How accurate was the item description?" is considered "bad" by eBay. It has the same effect on the seller as negative feedback.

http://pages.ebay.ca/sellerinformation/news/springupdate2014/sellerstandards.html#measure

[femmefan1946]

To be even clearer.

If you leave anything less than FIVE STARS for a seller, his ability to list and sell will be hurt. 

If you feel very 'meh' about a transaction, leave NO STARS. This is a Free Pass neither hurting nor helping him. 

If you are furious with your treatment leave ONE STAR. If a seller's DSRs drop below 4.3 his ability to list and sell is greatly reduced and his cost to list rises.