Luke Stillman, 16, from Walgrave, Northamptonshire, bought the apparently decommissioned Spanish 81mm mortar round from a website advertising specialist auction material. The boy was dismantling the round at his family home when it exploded, injuring his fingers and damaging the desk it was on, police said.
Luke needed hospital treatment for his injuries.
The website, Specialist Auctions, said the vendor had now been banned from trading.
They said: “We are very strict that the ordnance sold is inert.”
Detective Sergeant Andy Rogers, of Northamptonshire Police, said: “This incident could potentially have resulted in Luke sustaining a much more serious injury.
“As it was, his fingers were burned and the table he was sitting at was damaged.
“Anyone taking apart ordnance of any description should exercise the greatest care when doing so. Messing around with any ordnance is potentially very dangerous.”
Police are investigating whether any criminal offences have been committed over the round being live and available for purchase.