Handwritten letters reportedly penned by serial killer Ivan Milat are selling for hundreds of dollars online.

True Crime Collectives marketplace Supernaught listed the signed letters and envelopes on their website.

Watch in the video above: Ivan Milat moved from hospital to Long Bay

The US-based store is known for selling memorabilia from some of the world's most notorious killers, including Milat and Charles Manson.

A seven-page letter authored by the killer, who is currently on his deathbed, was up for sale on the website until Saturday morning.

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Written to an anonymous recipient he calls "Dave" in July 2014, he thanks them for a postcard before ending with his signature.

Above the date, Milat wrote: "The longer I live the longer is my sentence."

Letters penned by Ivan Milat are selling for hundreds online. Credit: Supernaught.com

The series of letters was available for just over $1,000 AUD, however they have since been removed from the marketplace.

A second piece of memorabilia, envelope signed by Milat and sent from Goulburn Correctional Centre, is still listed for $290.

Milat, 74, is dying from a range of throat and stomach cancers in prison.

Ivan Milat photographed in May 2019 leaving hospital. Credit: 7NEWS

Time is now running out if the backpacker murderer is to confess and give some closure to the families of his victims.

His white hair is balding and the killer's infamous handlebar moustache has also turned completely white.

Milat's nephew, Alistair Shipsey, has said his uncle's condition is "very bad" and he may only have a couple of weeks to live.

Milat was found guilty in 1996 of murdering seven backpackers whose bodies were found in makeshift graves in the 1990s.