Don't believe all the hoo ha about buyers wining 99% of the time, it's not true. Don't be intimidated by the process, it takes a week or more as Paypal needs to determine who is telling the truth. Unless it was leaking when you sold it, don't give him the $30 without a fight.
From here on out you should not communicate via PM. All your communications should take place in Paypal's dispute center. You should supply the link to your original sale thread and the link to his new sale thread (screen grab his sale just in case he deletes it). Calmly state your position that it did not leak and that there is no place it could be leaking where he says it is.