The firm was criticised for its "deficient" systems, which allowed youths to buy blades including a large carving knife, at some of its shops in London.
B&M Retail pleaded guilty in June this year to three counts of selling knives to under-18s.
At its Goodmayes Retail Park store in Redbridge on September 19 last year, a 15-year-old boy and 14-year-old girl were able to buy a set of four "very sharply pointed" knives, Barkingside Magistrates' Court heard.
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Just two days later at its Vicarage Fields store in Barking, a 16-year-old boy bought a three-piece knife set and at the same store on January 19 this year a boy aged 14 successfully purchased three knives.
District Judge Gary Lucie said the offences - which all involved volunteer youths making test purchases - were serious, and set the incidents within the context of London's high knife crime levels.
He said: "The stark fact is knife crime is at record levels across the country, particularly in London."
He added: "Clearly these offences were not deliberate nor were there serious or systemic failures within the organisation regarding the underage sales of knives. However, it appears to me that whilst systems were in place they were deficient."
He said it was concerning that the stores had not put in place measures to prevent such instances from occurring in what it accepted were "high risk areas".
The judge added that he had asked the company why it had not implemented measures such as putting knives in cabinets, away from young people, and said he had been informed "this would now be considered".
The company was fined £480,000 and ordered to pay costs and the statutory surcharge, bringing the total amount owed to £492,598.53.
District Judge Lucie said while the fine may seem large it is "approximately one day's net profit" for the company.
B&M Retail was given 28 days to pay.