Before billionaire businessman Warren Buffett started auctioning off private lunches to benefit the Glide Foundation, he was sceptical of the San Francisco charity where his first wife was volunteering.

But once Susie Buffett, who died in 2004, showed Warren the work Glide does for the poor and homeless, he was sold on the organisation - so much so that he's raised nearly US$16 million ($18.9 million) for the charity since 2000.

"It was one-on-one working with people society had given up on," Buffett said. "And experience showed society was wrong to give up on those people."

The 15th annual lunch auction started yesterday with a US$25,000 minimum bid on eBay and runs all week.

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The lunch auction has become an important source of money for Glide, which has an US$18 million annual budget. Glide's co-founders, the Reverend Cecil Williams and Janice Mirikitani, hope the lunch will draw another seven-figure price tag, but they also appreciate the exposure.

"I think it's amazing to have Warren Buffett as an advocate," Mirikitani said. The charity provides meals, healthcare, job training, rehabilitation and housing support to the poor and homeless.

Last year's auction winner got a relative bargain by paying US$1,000,100. Four of the previous five winners each paid more than US$2 million, and the 2012 winning bid of US$3,456,789 remains the most expensive charity item sold on eBay.

Buffett is confident this year's bidding will top the 2013 figure - "I think we'll beat it by quite a bit" - based on the limits prequalified bidders have set for themselves.

Buffett isn't quite sure why people are willing to pay so much for a private audience with Berkshire Hathaway's chairman and CEO, but he gives Glide part of the credit. The lunches typically last several hours, and Buffett tries to make sure the winners are satisfied.

The only limit on the conversation is what he might invest in next, but any other topic is open, including the billionaire's investing philosophy and his thoughts on philanthropy and inheritance.

"It goes all over the map," he said.