According to Mr King, there was also no agreement of the essential elements of the contract at the time the hammer fell.
But Barfoot & Thompson's lawyer Tim Rea said Ms Nguyen's brother and co-owner of the property, Vinh Hgoc, had good English and a business background, so there was no way he didn't know what was being signed.
"It is completely unconvincing that he would blindly sign documents given to him by a real estate agent," Mr Rea said.
"He is an extremely successful businessman and self-made man who has built himself an empire of businesses and properties. That he's inexperienced in property dealings is a charade.
"All witnesses for the defendants are making up completely unbelievable stories to justify a change of heart."
Ms Nguyen previously told the court she believed Mr Yap had "conspired" with the eventual purchasers.
Mr Rea said the serious allegations against Barfoot & Thompson and Mr Yap were unbelievable, didn't make sense, and were "all over the place".
Dr Glover and Mr Piner want the court to enforce their sale contract and award hefty penalty payments.
Their lawyer Simon Dench also said it was unconvincing that Mr Nguyen didn't understand what was being signed.
"The likelihood he didn't know what he was signing, when the auction was and the set price is slim at best," Mr Dench said.
"The auction and events that preceded it were unremarkable. They were ordinary events you'd expect an agent marketing and selling a property by auction would undertake.
"The plaintiffs had no knowledge of anything going on outside of that."
Dr Glover and Mr Piner's family-owned company SM & T Homes Ltd bought the house as an investment nestegg.
They paid a $44,000 deposit which remains in a trust account while the case is settled. They say they have lost tens of thousands of dollars in rental income and interest payments as a result of the stymied sale.
Justice Rebecca Edwards said the case came down to the legitimacy of the contract and whether it was enforceable. She reserved her decision.
The case
* Vietnamese brother and sister Thi Kim Chu Nguyen and Vinh Hgoc Nguyen listed their property at 42 Church St, Otahuhu, through Barfoot & Thompson.
* Auckland academic Dr Marewa Glover and husband Steve Piner bought the house for $450,000 in December 2013 after their pre-auction offer of $440,000 was accepted and the auction brought forward.
* However, the Vietnamese relatives later wanted to back out of the sale, and Ms Nguyen still lives at the property.
* Dr Glover and Mr Piner are suing, and want the court to enforce their sale contract and award hefty penalty payments. They have already paid a $44,000 deposit into a trust, and claim to have lost tens of thousands of dollars in rental income.
* Ms Nguyen claims to have limited English and had no understanding of what she was signing. She argued the agent had conspired with the buyers.
* Barfoot & Thompson has dismissed those claims as unbelievable and argued Mr Nguyen knew what he was doing.
* The civil dispute was heard before Justice Rebecca Edwards in the High Court at Auckland last week. Justice Edwards has reserved her decision.
