"It kind of hit home for me," he said.
The ad was a post by a man requesting a kidney donor for his wife. Calderbank decided to reply.
That was over the summer. Months later, Calderbank, 49, is preparing to donate one of his kidneys to Nina Saria -- formerly a stranger from Craigslist and now a friend.
The situation Saria and her family faces felt familiar to Calderbank.
Nearly 15 years ago, Calderbank posted a similar classified advertisement for his then wife, Jessica, in a local newspaper.
She struggled with kidney disease, and Calderbank wasn't a match. At that time, Calderbank says, placing an ad was an unusual choice. Organ donations were strictly anonymous unless they were from family members.
"I felt like no one was doing anything beyond putting her on a list," Calderbank told CNN by phone from West Berlin, New Jersey.
"This is all I've got," he said of his mindset at the time he placed the ad.
Jessica ended up receiving a kidney and a pancreas from a cadaver, but complications followed, and Calderbank grew all too aware of the struggles of those on dialysis. Hours upon hours spent in a hospital, in addition to the side effects of her disease, were trying for Jessica.
"She wasn't really living," Calderbank said. "She was just surviving."
Jessica passed away in 2011 after years of battling diabetes and kidney disease.
Calderbank remarried and, in some ways, moved on. But ever since his wife's death, he's wanted to help somebody.
An unexpected diagnosis
At first, Nina Saria thought the pain in her knee and her arm originated from a tough workout.
"It just felt like a muscle ache," she said.
But trouble breathing led to an emergency room visit and testing. In September 2014, Saria was diagnosed with end-stage renal failure from microscopic polyangiitis, according to the ad her husband posted.
The kidney disease had been lying dormant and largely symptomless. It meant that she had to start dialysis, and she began to search for a kidney donor.
Saria learned, though, that the wait list for her to receive one was some six to seven years long. So her husband, Kay, suggested the modern-day version of posting a classified ad -- a Craigslist post, seeking a brave person to donate.
"We are a family of four, me and my wife are 32, our son is 6 and our dog is 4. My wife is a very sweet woman with a big heart," the ad read. "If you are brave and find it in your heart that you want to save her life, I urge you to reply to this post."
Calderbank called soon after seeing the ad.
The Sarias and Calderbank met at Calderbank's home, where he showed them a scrapbook documenting his wife's journey through kidney disease and her treatment.
Calderbank delivered the good news to Saria; he wanted to give her his kidney. Despite the odds against it, he was sure they would be a match.
"There is no way I saw your ad by accident," Calderbank remembers telling Saria, who lives in Egg Harbor City, New Jersey.
Saria couldn't believe what she was hearing.
"These words were something I waited more than a year to hear," she told CNN.
Saria was excited but wanted to keep her expectations in check. She had had hope before in others who had made similar promises and then backed out.
Calderbank embarked on what amounted to three months of testing -- everything from blood tests to speaking with psychiatrists and lawyers.
He found out last month he was cleared to go forward with his donation.
Friends by chance
Since then, Saria and Calderbank have been counting down the days together until their December 1 operation date.
Their families have become close and often meet for dinner. Once, they met for chicken wings.
"I feel very close to them," Saria says of Calderbank and his wife, Susan. "I feel like I knew them all my life."
"This brought us together," Calderbank said.
Both say that Saria has asked Calderbank what she can do to pay him back. But he has been steadfast in his reply.
"I'm doing this not for you to pay me back, but for you to pass it on," Calderbank said.