If it’s a threat, just totally ignore it.
If you get a £50 solicitor’s letter requesting you to remove the negative feedback, and threatening to recover the costs… also just totally ignore it.
If you get more than that, then I’d leave negative feedback indicating that they threaten legal action, and report to ebay.
IANAL, but I know many people that have received such letters from solicitors, and most framed them as a souvenir.
edit: I lie – one acquaintance did do the “see you in court” thing (since his reputation had been harmed by the seller’s abusive legal threats), and won a undisclosed damages in excess of £19,999.