It was not on the market, but the pensioners decided to downsize and free up capital to help their own family.
They obtained a valuation then negotiated a price of $1.95 million and sold privately to Xie and Li in July, with settlement to occur on January 27 this year.
Xie and Li told the couple they planned to live in the house with their extended family from China.
But in February, just two weeks after settlement, Millar received a call from her sister-in-law saying the house was on the market.
Shocked, Millar attended an open home. The house was empty and just as she had left it. But a bunch of flowers she had bought for the purchasers as a welcoming gift was displayed on a bench.
Millar eventually learned the property had been on-sold through First National for $2.48 million - more than twice its capital valuation - giving Xie and Li a $530,000 windfall.
The on-sale is yet to settle and the new owner is unknown.
Millar said she and her husband felt like "suckers" and in hindsight wished they had used a real estate agent.
The extra money could have helped their daughter, who was moving out of Auckland, into her own house.
"We could have done so much with that extra money to help our families, and our families do need help."
Millar had tried to be "philosophical" about the on-sale, because it had been so upsetting.
"Otherwise you would just be consumed."
Li told the Herald she and her husband had intended to live in the house but had trouble raising finance and were forced to sell.
She felt bad about what had transpired but had not set out to profit on Millar's home.
While the mark-up was more than $500,000, their actual profit was "not huge" after paying capital gains tax, agency fees and steep interest payments on loans.
"I don't want to upset Dianne but ... we can't lend the money.
"We tried our best and tried so many ways."
First National agent Raymond Kwan said his vendors were very happy with the $2.48 million sale price, which he described as fair.
The buyer was a New Zealander who would "possibly" live in the property.