Boothroyd said: "We don't need to sell - especially to a club who we might be changing places with.
"I want him to stay so he stays. That's it. I don't need any money so there is no fee for Marlon."
King missed most of last season through injury and was badly missed by Boothroyd's side as they were relegated from the Premier League.
The 27-year-old has scored 11 goals this term and with Watford attempting to get back into the top flight, Boothroyd has no intention of letting him leave.
Boothroyd added: "In order for a player to move I've got to want him to go, the player has got to want to leave, the money has got to be right and there has got to be a replacement.
"None of those four criteria are being met."