500W is too small for an S3, I'm surprised you haven't melted it down yet. A good rule of thumb is to not exceed 80% of the rated power of the psu.
Unless it is under clocked I would be afraid to buy it personally. I wonder if he is running through 4 or 2 PCIEe to. I am surprised it has not had problems either and run under that unless psu is wrong or under clocked.
But I would be interested to hear more of the story.