Do they just send you a bill or does law enforcement get involved?
I'm not sure I understand your question?
You are exchanging bitcoins for Paypal. The system creates a unique receiving address for each customer. After the first confirmation the system prompts us to make payment to the customer. The payment is then manually sent to the customer from the company account.
I think the question was: "Assuming you sell BTC in exchange for fiat currency via PayPal, and you then remove those funds from your PayPal account, and THEN the buyer reverses the PayPal transaction through a chargeback or other means, you would be left with a negative balance in your PayPal account. If this happens, what method if any would PayPal use to reclaim their money from you?"
Assuming I've got the question right, my understanding (based on personal experience) is that they will reverse the transaction leaving you with a negative balance in your PayPal account. If you have a linked bank account or credit card, then I believe PayPal can bill that after a set period of time. In my personal experience, they just sent me a couple of notices that a payment had been reversed and I had a negative balance - the next time I received a payment, they ate enough of it to offset the negative balance automatically.
I don`t know what PayPal`s specific policy would be, but most likely collections would become involved rather than LE. It`s going to be a civil matter unless they were somehow able that you were perpetrating a fraud or something. In any case it`d be inconvenient and they would certainly shut down your account in rather short order if you`ve pulled your money and sit there with a negative balance for an extended period of time.
I`m pretty sure there are others with more extensive personal experience on this who can tell you exactly what PayPal`s Standard Operating Procedure is on this one. I would also guess it varies from jurisdiction to jurisdiction. My specific example is for UK / US which seems to be basically the same on this matter.