You would probably end up getting a dispute. Paypal would hold those funds from your account, and you would need to prove that the transaction was legitimate and that the customer received what was agreed to them. If you can prove this, you get to keep the money. If Paypal rules in favor of the customer, the money is transfered automatically to them.
Please clearify the question: 1) has your account balance become negative? 2) did a buyer fill a dispute and you don´t want to pay it back? Answers: 1) if your account balance became negative and the account is in your real name I would highly suggest to get the balance positive or min. zero again. Otherwise you will get some nice letters from PayPals lawyers and this will be MUCH more expensive then just getting the balance positive again. If the account is in fake name it´s your decision: pay back PayPal from your own pocket or just leave it this way. There is nothing what can happen to you. 2) if a buyer filled a dispute and you don´t want to pay it PayPal will decide on the outgoing of this. Maybe they will decide for you, maybe for the buyer. Depends from case to case