Ian Sinclair, 48, of Gorleston, Norfolk, became the first estate agent to fall foul of the Property Misdescriptions Act, which came into force in April. The Act was designed to prevent the public being misled by property descriptions.
Mr Sinclair admitted making a false statement indicating that the price of a property was pounds 22,500 when it fact it was pounds 25,000.
Malcolm Partridge, for the defence, told magistrates at Great Yarmouth, Norfolk, that a new receptionist at the estate agent's office in Gorleston had forgotten to change the advert in the window after the seller raised the price.
A prospective buyer spotted a newspaper advertisement for the property at the lower price and went to Mr Sinclair's office.
But the property details she was given contained the altered price of pounds 25,000. She later complained to the Norfolk Trading Standards department.
Mr Sinclair, an estate agent for 30 years, was fined pounds 500 and ordered to pay pounds 570 costs.