I hope I'm wrong but to the best of my knowledge the previous owner should have deauthorized it before selling it. According to Sony to authorize it to a new user it first has to be deauthorized by the previous one.
I don't know how to do that by the second owner when the first owner no longer has possession of the reader & can't deauthorize it. Did you try searching the forums here or Googling the answer? Good luck to you!
Try this from the Mobileread Wiki. It's instructions for a hard reset. Follow the instructions for your model.
https://wiki.mobileread.com/wiki/Sony..._.28PRS-300.29