If you paid by bank transfer, the best you could hope for here is a chargeback dispute, your evidence would need to be substantial.
You have next to no rights as a buyer in private transactions. The item is for all intents and purposes sold as seen.
The idea is you’d have to prove the buyer deliberately misrepresented the item, realistically though your bank will likely side with you if you just show them evidence of the sale, the fault, and the seller’s non-compliance.
Beware though, some banks will charge you a ยฃ100 excess for claims that turn out to be fraudulent if you didn’t do your due diligence beforehand.
If you paid with cash, legally you can do absolutely nothing. You can try various tactics to get the money back but frankly, it will almost certainly lead to nothing.