Home sellers could lose tens of thousands of pounds when using quick-sale estate agents to find a buyer, Trading Standards has warned.
It is feared that scores of homeowners are falling prey to companies which end up significantly reducing the market value of their homes, sometimes without their permission, with the aim of achieving a speedy deal.
Lowering the sale price of a property without the seller's permission is illegal and can result in up to two years in prison as well as an unlimited fine.
National Trading Standards investigator Alison Farrar said the watchdog is currently monitoring a handful of estate agents offering this service.
But she warns there could be more which are operating under their radar and recklessly leaving people selling their homes out of pocket.
"I would imagine there are quite a few more we don't know about," she told BBC News.
One home seller, Marianne Phillips, told Radio 4's Money Box programme that the company she used to sell her £250,000 three bed semi-detached property reduced the asking price by £20,000 without her knowledge.
She ended up abandoning the quick sale company and relisting her home with a traditional estate agent.
Ms Phillips said she was "absolutely devastated", adding: "The equity I've built up in the house, they've just potentially wiped a lot of that out by reducing it by £20,000. It could have quite a dramatic effect on my future".
Consumers who think they may have been dealt with unfairly by a quick sale estate agent can contact National Trading Standards or one of two government-approved organisations: the Property Ombudsman Limited and the Property Redress Scheme.
Mark Hayward, chief executive of the National Association of Estate Agents Propertymark, says the safest way to secure an estate agent is look for one which is regulated and has professional indemnity insurance.
"I would warn people to make sure they've read the small print and to not be pushed into any price they're not comfortable with," he told BBC News.
"Your house is your biggest asset, don't be rushed into making any decision."