The shop assistant was the daughter of the licensee, said officers who were working in conjunction with Harrow Police Neighbourhood Teams.
The sting was part of the latest wave of test-purchasing operations across England and Wales.
The licence holder is liable for any infringement, after which the licence could be reviewed and revoked. Trading Standards officials refused to disclose the identity of the retailer.
Eleven off-licences and 23 pubs in the London Borough of Harrow have been visited by test purchasers in the past month.
Three off-licences fell foul of licensing regulations.