The seller, believed to be Waldeck, says the "one-of-a-kind 3.56 carat Ceylon sapphire gem and diamond ring has been certifiably valued at $19,150."
The seller adds that the ring is in "perfect condition with no marks or scratches" and describes it as having been custom-made from Wishart Jewellers.
Ali and Dave are believed to have begun dating soon after she appeared on Tim Robards season of The Bachelor in 2013.
In 2015, Ali gushed over David and revealed she had no hard feelings after Tim chose Anna Heinrich over her as the winner of his series.
"Tim absolutely led me to Dave and he is the most perfect man in the world," she told news.com.au prior to her engagement.
She added: "The Bachelor was my hurdle and my challenge and something I learnt from and made me more of who I am today and Dave is the one for me."
Dave proposed in June 2015, after he and Ali enjoyed a romantic trip to Europe together.
Earlier this year, New Idea exclusively revealed the salacious details behind Ali's romance with Dave.
According to a close friend, Dave and his older brother Steve stopped talking for three years after Steve, who dated Ali first in 2013, found out the blonde bombshell was sleeping with his younger brother Dave behind his back.
“Ali started dating Steve while working for him at Sunlounge Tanning Salon,” a friend of the brothers revealed, adding it wasn’t long before their relationship turned sour.
“On Ali and Steve’s first date, the pair went out and Ali ended up making out with five different girls at Mr Kim’s nightclub in Adelaide,” the friend, who does not want be named, continues.
“One of the girls even had a boyfriend who got pretty angry and they had to leave. The next day Ali was really mortified and kept apologising.”
As time went on, things went from bad to worse for the pair.
“She would often turn up unannounced at Steve’s apartment after a big night out,” the pal says.
“They got along well when there was no alcohol involved, but Steve really struggled when Ali would turn up drunk most weekends waking him up and making a scene. She would send hundreds of texts and ring his phone until the battery ran out. He even had to disconnect his doorbell because she just pressed it for hours.”
Eventually, Ali and Steve’s relationship ended when Ali accepted an offer to star on The Bachelor with Tim Robards as the leading man.
“They both agreed Ali should go on the show to chase her dreams of fame,” the friend explains.
Then, just days later, Ali bumped into Steve’s younger brother Dave at a bar.
“She told him he could buy her a drink if he liked what he saw,” the friend explains.
Dave and Ali began a secret relationship, before making it public weeks later.
“Steve was furious when he found out. He knew Ali was just doing it to get back at him and he didn’t want his little brother caught in the crossfire.”
But after some heated arguments, Dave said it was too late and that he loved Ali – and after 18 months he popped the question.