Dean Holborn, who owns two convenience stores in Redhill, Surrey, said retailers could not afford to let their guard down for one second.
His store was targeted just as it was about to close. The manager, who had already turned out the store lights and was busy closing the chiller blinds, did not see the teenage test purchaser enter.
The alcohol sale was made by a new member of staff who had only started work that afternoon, and who had not been left unsupervised at the till until that moment.
"The store was almost dark and I really do feel that this was not done by the book," Dean said. "We have always taken this issue incredibly seriously, so to be caught out this way was absolutely horrifying. Our refusals book goes back 14 years, and this has never happened before. It just goes to show that you have to constantly remind staff to be vigilant."
The employee was cautioned by police and handed a £90 fixed penalty notice.
The sting occurred in the same week that a Labour backbencher called on the government to show retailers greater respect for the "tough job" that they do.
David Williams MP said there was too much responsibility on the shopworker and not enough on the underaged person who is attempting to buy.
"I want the government to run a high-profile campaign making it clear to youngsters that it is an offence for them to attempt to purchase alcohol underage," he said.