The writer and activist trended on Twitter after the account @VeryWhiteGuy shared screenshots of an email from King.
In the message, King expressed his sadness at Boseman’s death then asked his followers to buy his book, Make Change.
“I hope you are hanging in there. Yesterday I sent an email checking in on everybody – and just a few hours later we learned of the death of Chadwick Boseman,” the email reads.
“Life is so very fragile. Tomorrow is not promised.”
Boseman died, aged 43, from colon cancer on Friday 28 August. The actor was diagnosed in 2016, but chose to keep his illness and treatment private.
In the same email, King claimed that, for the past six years, people have been asking him how he uses his life, skills and energy to “impact and change the world’.
“My book, MAKE CHANGE, is a 272 page answer to that question,” King wrote.
The account @VeryWhiteGuy accused King of attempting to profit off Boseman’s death, in a post that received more than 12,000 retweets and 35,000 likes.
“Shaun King is using Chadwick’s death to sell books, he claims don’t make him any money. I’m so tired of SK profiting off of Black death,” he wrote.
“Not Shaun King using Chadwick’s death to sell his book?! What actually does this man offer if not proximity to exploitable black suffering and death?” tweeted writer Zoe Samudz.
“I honestly need someone to explain like I’m a child because it’s been too many years of this now and it’s patently insulting.”
King claimed that he no longer makes any money from sales of his book, and insisted he had no regrets about the email.
“I literally do not make a penny off of sales for my book. Nothing. Got paid a year ago to write it,” he tweeted.
“I had a mass email pre-scheduled to go out this morning about my book. I updated the email to also mention the death of Chadwick. And I don’t regret it. At all. There’s that.”
However, King’s claim that he does not make any money from book sales prompted more questions.
“Why does Shaun King keep expecting people to believe that he got paid upfront to write a book and is not getting a single penny thereafter?” wrote Imani Gandy, an editor at Rewire.News.
“It’s ludicrous. He honestly thinks people are ignorant about how publishing works. Lol.”
Dr Rebekah Kavanagh accused King of lying and repeated the accusation that he was attempting to profit from Boseman’s death.
“Shaun King, this is a lie,” she tweeted.
“This isn't how book deals work. What you got paid is an advance. As books sell, your advance gets paid back. Once it is paid back, you start to make more money on sales.
“You were trying to profit off Chadwick Boseman's death and it's disgusting.”
Boseman's Avengers co-stars, including Letitia Wright, Samuel L Jackson, Chris Evans and Mark Ruffalo, have been paying tribute to the actor on social media.