Gurtej Randhawa had a package delivered that he thought was a remote-detonated explosive device and was subsequently arrested in May 2017, the National Crime Agency (NCA) said.
The 19-year-old had attempted to buy a vehicle-borne improvised explosive device but the NCA replaced the package with an inert dummy device before it was delivered.
Randhawa, of Grove Lane, Wightwick, Wolverhampton, had previously pleaded guilty to attempting to import explosives and was found guilty of maliciously possessing an explosive substance with intent to endanger life or cause serious injury on 7 November.
Two women, aged 45 and 18, were arrested at the same time but were released without charge.
Tim Gregory, from the NCA's Armed Operations Unit, said: “The explosive device Randhawa sought to purchase online had the potential to cause serious damage and kill many people if he had been successful in using it.
“He was not involved in an organised crime group or linked to terrorism, but is clearly an individual who poses a significant risk to the community.
“Identifying people like Randhawa - who seek to access illegal firearms and weapons - is a priority for the NCA and we will not stop in our efforts to make sure they are arrested and held accountable for their actions”.
Randhawa was remanded in custody and is due to be sentenced on 12 January next year at Birmingham Crown Court.