What is happening?
It sounds as though a buyer has raised a credit card chargeback that they did not authorise the PayPal payment. This may be genuine, or it could be the buyer lying to get a refund, which is a known scam.
It's odd that you've heard from eBay before hearing from Paypal. Especially as they're offering to return your selling fee after such a long time. Are you sure that this isn't yet another new scam or phishing email, trying to trick you into giving away your password? I hope you didn't click on any links it provided? Is there a copy in your messages in "My eBay", which there should be if it's genuine.
If it's genuine, the next thing you can expect is contact from PayPal requiring you to provide proof of posting the item. If you can do this, PayPal will indemnify you from any liability arising from the chargeback. If you can't, you will be required to return the buyer's payment in full - and pay an admin charge. You can read full details in PayPal's user agreement.
This is why it's so important to keep postal proofs for at least a year, and why accepting PayPal is so risky when an item is collected in person.
