People were able to pre-order the flamethrowers for $US500 each, and could buy an "overpriced" fire extinguisher with a "cool sticker" for $US30 as well.
"Obviously a flamethrower is a super terrible idea. Definitely don't buy one," Mr Musk said on Twitter.
But by Monday (local time) the entrepreneur, who is also the CEO of car manufacturing firm Tesla, said he had sold 10,000 flamethrowers.
That meant The Boring Company cleared at least $US5 million worth of its new product.
Mr Musk's idea to start selling the bizarre product began as a joke in December when he pledged that if The Boring Company sold 50,000 limited edition baseball caps, it would start selling flamethrowers.
But the dangers of selling a fire-emitting device, and Mr Musk's mischievous marketing tactics have raised questions about the legality of selling such a weapon online.
"ATF [Bureau of Alcohol, Tobacco, Firearms and Explosives] says any flamethrower with a flame shorter than 10ft is A-ok. Our design is max fun for least danger. I'd be way more scared of a steak knife," Mr Musk said on Twitter.
While in Australia it is illegal to import or own a flamethrower, in the United States, private ownership is not restricted by federal law and the devices are legal in most states.