The horrifying moment was captured on film at 9am on a Sunday morning - an hour before shops in Scotland are allowed to sell alcohol.
Father-of-three Sanjay Singh had refused to sell the tonic wine to the man, who was dressed in a unicorn onesie, and the young woman he was with, on the grounds that it was against the law.
Mr Singh, 43, believes the two had been on an all-night drinking binge and wanted to continue partying through Sunday morning.
But the pair, aged in their mid-20s, became aggressive and grabbed two bottles of Buckfast, which they had demanded, as well as wrecking the store.
In the CCTV footage the young woman can be seen spitting in Mr Singh's face as he ushers them to the door.
Mr Singh said: “They wanted Buckfast.
“They were high or had been drinking.
“People want their alcohol and the only thing stopping them is the shopkeeper.
“They had been up all night, they were in their 20s - old enough to know better.
“They got angry because I wasn’t going to serve them.
“They were saying ‘I want what I want’.
“I was just asking them to leave the store.
“There were three elderly customers in the shop in their late 70s.
“It was Sunday morning - I tend not to get anyone under the age of 50 on a Sunday morning.”
The confrontation happened at the Park Farm Post Office branch of USave, in Port Glasgow, on July 5.
Mr Singh said trading during the coronavirus crisis has been challenging but this incident had been particularly upsetting.
He says he was lucky the saliva hit him on the chin rather than full in the face.
He added: “It is disgusting at any time, but especially when there is a virus and a pandemic going on.
“We decided to stay open because there’s not enough shops in this area.
“It was very hard for the first few months.
“As a company we thought it was the right thing to do.
“I just want to go to work, that’s what I want to do every day, but I don’t go to work to deal with this.
“It is bad outside the pandemic, but this was in the pandemic.”
The shopkeeper said he had not been tested for Covid-19 and had not developed any symptoms of the illness.
Investigations into the incident are ongoing.
A spokesman for Police Scotland said: “Around 8.55 am on Friday, 5 July 2020, police were called to an incident in a post office/convenience store in Arran Avenue, Port Glasgow, in which the shopkeeper was assaulted and a quantity of alcohol was stolen.
“Enquiries are continuing.”
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