Giving evidence from a wheelchair, Dr Percy Thompson-Hancock claimed that despite his protestations the pair openly removed the canvases from his lounge wall.
London's Southwark Crown Court heard Michael Openshaw and Robert Barrett left pounds 10,400 cash "sitting on a table" for Children on a Promenade, and a few days later sent a further pounds 6,000 for A Family of Three.
"At first glance, a sale had taken place," said Duncan Atkinson, prosecuting. "But it is the prosecution case that no valid sale had taken place."
The court heard that they were later auctioned off at Bonham's for a total of pounds 78,000, before ending up in a Bond Street art gallery in central London.
They were spotted by the doctor's granddaughter and the police informed.
Openshaw, 51, and Barrett, 54, both from Hove, Sussex, deny stealing the two Lowrys in November 1996.
Dr Thompson-Hancock told the jury how the pair had visited his house on a number of occasions over the previous five years. During that time he had sold them a number of items. But he maintained that although they seemed "nice and sociable and appeared genuine ... I didn't really trust them".
Mr Atkinson said that when the men were interviewed by police Barrett said he had bought the two paintings quite properly and believed the pounds 16,000- odd paid for them reflected their correct value.
Openshaw, said the barrister, told police no one from the family had complained about the transaction and insisted the "situation was ridiculous".
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