Yes, it's true, ratings can be retrospectively changed. I can see how, in genuine circumstances, a mistake from a seller can be made but as long as it is rectified all is good and a good rating awarded. In your case you got refunded by PayPal via Ebay, in my experience the system works very well, although the new system where Ebay has dropped PayPal for sellers is interesting. I am sure offering money for a good rating is out of bounds and imagine if you had a component problem and invoked warranty 😱