At around midday on Saturday, Anne Mocatta said two men who were 'strongly built and with dark hair' offered to fix and clear out the guttering on the roof of her home in Rectory Road, Worthing.
But instead, Anne said they forced her to hand over £280 for work they never did, and walked her to the Tesco Express store a few doors down to get out another £100.
Once they left, the great-grandmother also realised that while one of the men watched her, the other rifled through her belongings and stole £30 she had raised to help leprosy sufferers abroad through her church.
She said she felt 'mesmerised' by them into doing what they said, and had been left feeling 'violated'. She said: "You know what hurts the most? That they took my leprosy mission money.
"It is so sad that people can be so evil that they would do something like this."
The retired nursing sister praised her neighbour, who reported the incident to the police and was there with her when the pair said they would return to fix her roof at 9am on Monday. They did not come back.
Her friend Major Tom Wye, Worthing Borough Councillor, posted on Facebook to warn people of the scam. He described Anne as 'the salt of the earth' and added that police officers who came to see her after the crime was reported were so taken aback by her story that they gave her £10 towards her leprosy mission money.
He said: "I would like to thank the officers, because they have made a hell of a difference to her."
It was the second time Anne had fallen foul of a scam. Last year, she said she paid two other men also masquerading as builders £280 for work they did not do.
She believed these men had told the pair who recently conned her that she was 'an easy target'.
She said: "The saddest thing about being a Christian is that you trust people."
The pensioner warned other older people to stay vigilant and not accept work from traders who knock at their door.
"It is very, very scary when you are past your sell by date like me," she joked.
A police spokesman said: "Anyone with information is asked to contact Sussex Police online at https://www.sussex.police.uk/contact/af/contact-us/ or call 101, quoting serial 1325 of 09/02."