An online bidding war is taking place for a five-year-old Wispa bar.
Wispa bars were brought back in October as a trial
Student Rebecca Wells, from Stoke-on-Trent, found the out-of-date Cadbury bar down the back of a sofa.
She has put it up for auction on the eBay site to raise money for the charity LAM Action, which funds research into a fatal lung disease.
The Birmingham firm stopped making Wispa bars in 2003. It brought them back as a trial in October following an internet campaign.
So far bidding for the bar has reached more than £200 - hundreds of times its original sale price.
Ms Wells said she initially wanted to eat the bar - which was best before 28 April 2004 - when she found it.
She said: "It kind of went through my mind but I knew it was probably going to be years out of date.
"I didn't know what to do with it. I was looking for things to put on eBay when I found it so I thought I might put it on eBay and see what happens."
Peter Bonner, who led the Bring Back Wispa campaign, said the auction might appeal to people who want a memento from their childhood.
He said he thought it might sell for as much as £50.