From the Trading Standards website:
What rights do I have if I buy goods privately, from an individual?
Not many! The Sale of Goods legislation does not wholly apply, although goods will still have to meet any description given by the seller or you can take action for breach of contract. You should watch out for traders who pretend to be private sellers in order to avoid statutory liabilities - if you suspect a private seller is really a trader contact your local trading standards office, they will be able to take action under Fair Trading Act powers.