Sometimes, there is no money in the seller's account and nothing for Paypal to recover from them.
This seems to be when they do nothing.
After all, they aren't going to pay you out of THEIR monies.
If a seller is stupid enough to give paypal a checking account link, then paypal can always recover money because they will take it from your checking account.
Even if you get a chargeback from a buyer that is fraudulent. A quick check of google will turn up cases where paypal has frozen funds in sellers accounts for months with little explanation, where paypal gave a refund to a buyer with no requirement for the buyer to send back the goods - they end up with your product AND their money back, etc.
Hearing from people who say "I've used paypal for years without any problems" doesn't really address the poor people's plight who get hit by this buzzsaw.
And, if you sell and take paypal, you could be next. You have little to no protection.
If you buy and use paypal, use funds from a credit card. It's the only real protection you have.