I reacted on it saying Polly Jean, thought she would just write PJ since it is her artist name. Now when I look at other signed items I see that it's the same thing everywhere. So it maybe is real?
I couldn't say for sure unless I could hold the guitar in my hands and compare it to another known autograph. But, it does look like other autographs of hers, particularly the capitals, and if you research the seller he seems to have a solid reputation. I wouldn't consider buying it, but I don't collect autographs. So, I don't know.