Khalid Ahmad, who runs A2Z Fireworks in Huddersfield, was convicted of the offence at Kirklees Magistrates Court after an undercover sting.
Mr Ahmad had been warned about his staff's conduct back in 2015 by Trading Standards, and was targeted by an undercover test purchaser in November 2016.
The boy, who was 15 years old, was able to buy six Moonshot rockets from one of the shop's employees.
Mr Ahmad was fined £500 and ordered to pay £1,500 in legal costs.
On the night of the Trading Standards operation, seven shops were targeted, but only A2Z staff sold goods to the underage purchaser.
In mitigation it was put to the court that Mr Ahmad had trained his staff and warned them to be vigilant however no documented evidence, nor training records, of this had been presented to Trading Standards.
David Lodge, head of West Yorkshire Trading Standards said:
“Fireworks can be a fun family activity when used responsibly however they can be extremely dangerous if falling into the wrong hands, including those of minors. The service will continue to take legal action against those that act irresponsibly and sell fireworks to people underage.”