“I heard that people are lining up to scan the code when they enter government buildings, so I think this will make it easier for me and also for others,” he said.

Most people entering government buildings and other venues now must use the app on their phones to scan a QR code. Only children under 12, those over 65, and people with disabilities are exempted and will be allowed to fill in forms before entering.

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The mandatory use of the app is regarded as a necessary first step towards meeting Beijing’s concerns over Covid-19 pandemic controls as Hong Kong works towards reopening the border with mainland China.

The change led to tens of thousands of Hongkongers, including the Yaus, rushing out to buy cheap second-hand phones that can download the app.

Second-hand smartphone vendors have raked in hundreds of thousands of Hong Kong dollars clearing their inventories. Some charged as much as HK$200 to download the app, which is free, for customers who did not know how to do it themselves.

At the DIY Telecom shop in Sin Tat Plaza, a salesman who gave his name as David said second-hand phones costing HK$400 to HK$700 were the most popular over the past two weeks.

Cheaper choices were available at the Apliu Street flea market in Sham Shui Po, where dozens of stores were pushing second-hand stocks labelled “Leave Home Safe app phones”.

Niko, a 28-year-old salesman at JK Telecom, said sales shot up 40 per cent over the past fortnight, and the store made HK$10,000 to HK$15,000 from hundreds of deals.

He said more than 50 brand new phones priced at HK$399 sold out last week and the shop had to stock up quickly with second-hand phones from the mainland.

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Niko said the shop did not raise prices – which ranged from HK$399 to HK$699 – but seized the opportunity to clear its unpopular models.

“Many second-hand phones have been stocked for over a year, because they either run too slowly or are of poor quality and inconvenient for frequent use,” he said.

The shop sold all phones with the app installed. Niko said after clinching a deal, he showed customers how to use the app to scan a QR code, and charged a service fee of HK$50 to HK$100 for the installation and his one-minute explanation.

“Ninety per cent of the stores here charge about the same amount,” he said.

At JK Empire, another store at the market, Peter Lau Sing-ying, 30, said his second-hand phones were priced around HK$500, including the pre-installed app. It sold more than 200 phones in recent days.

A woman in her fifties came to his store on Thursday afternoon and checked out a HK$599 phone but did not buy it.

Preferring to remain anonymous, she said she had been looking for a second-hand phone for days, but could not be sure that the cheap ones were of good quality.

“Now it’s too troublesome to scan the code everywhere,” she said.

The wider use of the app has resulted in some concerns about privacy, but the Office of the Government Chief Information Officer assured the public that there was no need to worry about the “Leave Home Safe” app.

In an email reply to questions from the Post, it said the app did not require users to register any personal information and did not have any tracking function.

Venue check-in data and Covid-19 exposure notifications, if any, would be encrypted and saved in users’ devices only.

Such data would not be uploaded or transferred to the government or any other systems, and would be deleted automatically after 31 days, it said.