Ms Nguyen told the court she asked Mr Yap twice to cancel the auction because the offer price was too low and that she was not told what the reserve price meant.
"I was not happy with that price but I signed because Ricky said it was only a starting price. He never explained to me that if no one bid then that is what the house would be sold for.
"I believe the agent and purchasers conspired to stop me from cancelling the auction and ensure the property was sold to them."
Ms Nguyen told the court she rang Mr Yap the night before the auction in a bid to call it off but he insisted it had to go ahead as he had four potential buyers.
On auction day she was shocked to learn her house had been sold for only $450,000.
She felt "dizzy and faint" and recalls signing a document but said she did not know what it was.
"I can't recall how I got home after the auction. I think I signed a piece of paper but I don't know what kind of paper it was."
Breaking down in tears, Ms Nguyen also told the court she had listed the property for sale following the 2012 death of her eldest son in a car accident.
"Every time I walked into my son's room I cried. Everything in the house reminds me of my son. The pain became so big that I couldn't bear it any longer so I decided to sell the house."
However, Mr Yap disputed claims he was asked to call the auction off and says the first time he became aware the vendor no longer wanted to sell their house was a text message from Ms Nguyen several days after the hammer fell.
Mr Yap told the court he had presented the couple's pre-auction offer to the brother and sister and clearly explained the $440,000 figure would be the reserve price.
"I certainly did not tell [the sister] the Church St property would fetch much more than $440,000."
He said the sister had subsequently claimed she was unhappy with the price and that she had asked for the auction to be cancelled prior to it going ahead, and that he had refused to do so.
"That is not true."
He said Ms Nguyen also alleged he had colluded with the purchasers to ensure they bought the property.
"This is an astonishing allegation and is not true.
"I don't know Mr Piner or his wife personally and have not come across them in the past."
Under cross examination by the vendors' lawyer, Noel King, Mr Yap was asked whether the Ms Nguyen had tried to cancel the auction after an open home on November 30, 2013. My Yap replied that request had never been made.
"The night before the auction was held Thi phoned you and told you she wanted to cancel the auction," Mr King said.
"But you told her she couldn't because she would be in big trouble with the law.
"On the day of the auction neither Thi nor Vinh signed anything on this contract."
Mr Yap replied that Ms Nguyen signed the necessary documentation after the auction.
Dr Glover and Mr Piner are asking the court to enforce their unconditional sale and purchase agreement and award hefty penalty payments.
But the reticent vendors claim there was no written sale and purchase agreement and that they were misled by their Vietnamese estate agent.
Dr Glover and Mr Piner's family-owned company SM & T Homes Ltd purchased the house at auction in December 2013 as an investment nest egg with a $450,000 winning bid.
They paid a $44,000 deposit and say they shook hands with vendors Thi Kim Chu Nguyen and Vinh Hgoc Nguyen in Barfoot & Thompson's downtown auction rooms before heading off to arrange insurance.
Their loaned deposit remains in a trust account while the case is settled and they say they have lost tens of thousands of dollars in rental income and interest payments as a result of the stymied sale.
The case continues.
