From the BBC News:
The correspondence, written by Shelley and his friend and biographer Thomas Jefferson Hogg, was destined for a car boot sale before being identified.
The letters could fetch £30,000 when they go on sale at Christie’s in June.
Shelley and Hogg were expelled from Oxford University in 1811 for writing a pamphlet about atheism.
Both men were summoned before the authorities when the article was circulated. Shelley refused to co-operate and Hogg protested.