"if a seller has free delivery they automatically get 4/5 stars".
I have just completed some eBay feedbacks, and the only thing that I notice with free delivery, is the delivery stars are blanked out, with an explanation at the side. "Why can't I enter this rating- Free P&P automatically gives the seller a 5 star rate" (on how reasonable were the P&P charges?). If its free, then you might consider that the P&P charges warrant top marks?.
The other 3 tick boxes/lines for star rating, are available to rate the seller and includes how fast the delivery was, which is then for you to decide.