Not personally, although a friend of my Wife is currently being pursued by PayPal.
It’s a long and sordid story, She’s really an innocent party, but she sold a stolen item on behalf of her abusive ex-BF. The buyer was the original owner – she got arrested, bailed and ultimately released without charge – they put a warrant out on her ex-BF.
Anyway, PP returned the fee to the buyer, a little over £1k so not ‘small potatoes’ so she owes PP a grand. They said they will chase the Ex, but only if he’s convicted, the Police have all his details but aren’t actively looking for him, so unless he’s stopped in a car or does something else to be arrested he’s Scott free.
As for the mechanism, it’s like anything else – firstly they’ll send increasingly angry e-mails to you, then letters. Your PP account will be suspended and you’ll be banned from opening another (how much they can enforce that who-knows). After a time it will pass to debt collector – ultimately they’re powerless UNLESS they take you to court – if they secure a CCJ then they have more power to enforce the debt, even more so if they go to the High Court when they can instruct bailiffs.
As a rule of thumb no one is going to take you to court for less than £500, but more than likely over £1000 because of the costs involved (in man hours).
Anyway, in regards to the OP – if a seller doesn’t respond to the first e-mail start a dispute through the usual channels.