Under the new ebay rules a seller cannot leave you a negative feedback anymore. You can leave him what you want without retaliation.
As for the star rating again he cannot see what you leave so give them what you feel is fair.
Postage costs from hong kong etc- a lot of them charge a low price for the item but stupidly big price for the postage as they only pay ebay fee,s on the listing fee,s and final value of the item, not on postage, so they save on fee,s. Ony thing you can do about this is report them to ebay
http://cgi6.ebay.co.uk/ws/eBayISAPI....ss+postage+fee