I think you are the victim of a hacker. Its well known that hackers steal the password for paypal, buy things with it and the real owner of the account would then make a chargeback.
Your digital good would be ideal to steal because its even more difficult to track the hacker down.
So I think you have no chance to get the money back...
I dont know what to do to prevent this. Maybe write the buyer a email before sending out the software so that he can confirm the buy? So that the real owner get a email?
Really dont know...