The Russian Orthodox Church has expressed indignation at an attempt to sell a skull and bone allegedly belonging to Saint Philipp.
The advert for the remains appeared on a Russian website.
It described the relic as "remains of an Orthodox saint, in good condition, with an inscription on the cranium confirming the saint's name".
The Church has not said whether it thinks the bone and skull are real, but has described the ad as "immoral".
The seller, Boris Georgiev, a retired Soviet Army colonel, told BBC News he was "acting on behalf a friend".
According to Mr Georgiev, the 16th-Century relic was brought to St Petersburg at the height of the Bolshevik anti-religion campaign in the 1920s.