A BBC Radio probe into quick-sale firms claims some of them can lose sellers many thousands of pounds.
It says Trading Standards officers across the country have seen dozens of cases of people losing tens of thousands of pounds off the market value of their homes.
The Radio 4 Money Box programme says: “Some homeowners will accept a lower offer price to secure a speedy sale, and many firms provide a good service.”
But it adds that Trading Standards has “a few of these companies on their radar" at present.
The quick-sale firms typically say they can sell a home within seven or 14 days but the radio programme says it knows of many examples where people have had the offer price of their homes reduced, without their knowledge or permission, by tens of thousands of pounds.
It cites one case study - Marianne Phillips - who claimed the price she agreed with a quick-sale firm handling her home was £250,000. However, the firm allegedly dropped the asking price by £20,000 without her knowledge or permission.
"No way did I ever give my approval for that, I was absolutely devastated. It's a lot of money to reduce a house by without ever getting in touch with the person who owns it… 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” she tells the programme.
And the programme speaks with lawyer Lee Reynolds who says: “Dropping the price of someone's house is a potential breach of what's called Consumer Protection from Unfair Trading regulations. That's punishable by up to two years in prison and/or an unlimited fine. If it was done knowingly or recklessly it's still a potential breach.”
Mark Hayward, chief executive of the National Association of Estate Agents, told the BBC the safest way to secure an estate agency was to 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, Your house is your biggest asset, don't be rushed into making any decision."
You can hear the story here.