Official notices posted on the Ballard store greet customers, explaining its liquor license has been suspended for the November 19 violation.
A Liquor Control Board spokesperson said agents sent in an underage person with a valid ID showing he was under the age of 21. A clerk allegedly sold spirits to the minor.
The Ballard Bevmo store must pay a $500 fine and is prohibited from selling alcohol for five days.
This latest bust is raising new concerns about whether private retailers are doing enough to prevent sales to underage people.
"It's the community's responsibility to keep alcohol out of the hands of minors," said Amy Ezzo, director of the Washington State chapter of Mothers Against Drunk Driving. "Retailers are a part of the community."
The Liquor Control Board says its enforcement officers have tested all of the approximately 1,400 liquor retail outlets since voters passed privatization in June of 2012.
Compliance as of the end of 2013 was nearly 92 percent. That's about the same as when the state operated liquor stores, the spokesman says.
Ezzo of MADD says her organization works with the Liquor Control Board to raise awareness about alcohol sales to minors. That includes encouraging training for clerks.
"We certainly want to work to increase that compliance rate. That's not to say 92-percent isn't good. It's to say that 92-percent can be better."
Bevmo's corporate offices in California did not respond to an interview request.