A designer bag due to go on sale for charity for £5 is being touted on the auction website eBay for up to £175.
20,000 of the bags will go on sale at Sainsbury's in April
The creation by Anya Hindmarch bears the slogan "I'm not a plastic bag", and is aimed at encouraging people to reuse bags when they go shopping.
It is not officially launched until next week, but has already sold out in advance on the designer's website.
Now sellers are cashing in on demand by offering it on eBay at greatly inflated prices.
The bag was dreamt up by not-for-profit movement We Are What We Do, which tries to encourage people to use simple everyday actions to change the world.
They say the average Briton uses 167 plastic bags every year - 10 billion in total.
The reusable cotton bag was launched at London Fashion Week and handed out to a limited number of celebrities and fashion insiders.
It will go on sale in Ms Hindmarch's stores and on the We Are What We Do website on 20 March, while 20,000 will be available from selected Sainsbury's stores on 25 April.
But a number are being offered on eBay for £40 or £50 and even as much as £175, despite a notice posted by the makers warning potential buyers not to pay over the odds.
Kenny Jordan, consultant to We Are What We Do, said they were upset by what had happened and had been in touch with several sellers.
"We contacted some of the sellers by email saying, 'Would you buy Comic Relief red noses, sell them and keep the profit?'," Mr Jordan said.
"We did manage to get a couple of people to remove listings, but some said, 'Well we're just meeting demand.'
"Our main objection is the people who claim on their listing that it has already sold out and that the only way you can get it is by paying over the odds. Their buyers are being ripped off.
"People who say it is already sold out are at best being misleading and at worst telling downright lies."