The men deny conspiring to extort money for the painting's safe return
A solicitor has told the Leonardo da Vinci extortion trial how he fell "hook, line and sinker" for an undercover police operation.
Marshall Ronald, 53, said he took two officers, one posing as an art expert and another claiming to represent the stolen work's owner, "at face value".
He claimed the police had been "duplicitous and deceptive".
Mr Ronald and four others deny seeking £4.25m for the safe return of the Madonna of the Yarnwinder painting.
The artwork was stolen from the Duke of Buccleuch's Drumlanrig Castle in 2003.
None of the men on trial are accused of the robbery.
Mr Ronald told the High Court in Edinburgh he was put in touch with a man he knew by the alias John Craig by a loss adjuster dealing with the theft of the artwork.
He said Mr Craig claimed to represent the duke.
"I trusted him, I trusted him from the outset," Mr Ronald told the court.
"I genuinely believed they were who they said they were.
"I was taken in hook, line and sinker."
The Lancashire lawyer denied making any threats about the fate of the painting.
He said his one regret was "the cardinal sin" of raiding his law firm's client account for £500,000, which he believed would be repaid when he received his reward for returning the painting.
"I dreamed the dream and took the risk," he told the court.
The painting was stolen from Drumlanrig Castle in August, 2003
However, he criticised the police operation which had brought himself and his four co-accused to trial.
"The only people who have shown a lack of trust are the police," he said.
"They have been duplicitous and deceptive.
"They played footloose and fancy free with other people's property."
Mr Ronald told the court that the five men on trial had done what police had been unable to do and recovered the duke's painting.
"I think we did the right thing," he said.
"I am very proud of it and what we have achieved. I think it is very sad that we are standing here today."
On trial along with Mr Ronald are Robert Graham, 57, and John Doyle, 61, all from Lancashire, Calum Jones, 45, from Renfrewshire, and David Boyce, 63, from Lanarkshire.
They deny conspiring to extort £4.25m and an alternative charge of attempting to extort the money.
The trial continues.