A Sydneysider who is selling their ballot paper on the same-sex marriage postal survey has been warned they face possible jail time or a $2000 fine, with an independent senator saying it's making a mockery of the exercise.

The eBay listing, which has since been taken down, was published on September 18 and titled, “Buy my vote”.

The bid started at $1500.

“What is this plebiscite worth to you,” the seller writes in the ad description.

“The reason I'm selling my vote is because either way I don't care, but thought there are people who do.”

SBS/eBay

The seller promises part of the proceeds of the auction will go to “help kids battling cancer”.

The Australian Bureau of Statistics, which is conducting the survey, told SBS World News it considers any submission of a survey response that’s been bought or sold a criminal offence.

“The offence against the Census and Statistics Act 1905 carries a maximum penalty of $2,100. The Criminal Code offence carries a maximum penalty of 12 months imprisonment,” a spokesman said.

The ABS has contacted several online marketplaces including eBay, Facebook, Amazon, Alibaba, and Gumtree.

“To date eBay and Facebook have confirmed listing survey forms or survey responses for sale would not comply with their policies and they will block and remove any such listings,” the spokesman said.

Independent senator Nick Xenophon says such conduct should be made illegal.

“I mean, the fact is, any attempt to game the survey, any attempt to distort the results of this important survey, must be met with appropriate legal safeguards otherwise it makes a complete mockery of it,” he told reporters.

Same-sex marriage advocates called on all Australians to respect the postal survey process.

"Because it is about the value and worth of gay and lesbian Australians and whether they are entitled to the same aspirations as their friends and family members," Clint McGilvray from the Equality Campaign told SBS World News.

Acting Special Minister of State Mathias Cormann has been contacted for comment.