The quest for an online "bargain" is making scam victims press ahead with a purchase even though they realise it may be a con, research has found.
More than a quarter (27%) of Britons have been the victim of an online marketplace scam, losing £63.76 on average in each incident, according to a report from website Gumtree.
Popular categories targeted by online fraudsters include items for sale, vehicles, jobs, services and property, the research found.
The desire to grab what looks like a cheap deal often overrides people's instincts when shopping online, according to the survey.
More than a third (35%) of scam victims who thought an advert may be a fraud still went ahead.
The top reasons for people falling victim included that something was perceived to be a good deal, while one in five (19%) were willing to take the risk because they really wanted something.
In some cases, a fraudster appearing to be kind and trustworthy - for example, offering to travel to the victim's home to make a transaction - lulled them into a false sense of security.
Items subject to scams tended to be slightly cheaper than others available for sale, but not so keenly priced as to arouse suspicion - making victims think they had spotted a good deal.
The embarrassment factor meant that, after being scammed, one in six (15%) victims did not tell anyone.
Researchers also showed eight adverts to people and asked them to identify the scams. Only 7% correctly identified all the bogus adverts - while 93% of people could not spot all the scams.
Those who spotted fake adverts said giveaway signs included spelling mistakes, pictures looking "dodgy", a lack of detail and factual errors in the description.
The findings from the Psychology of Scamming report coincide with Scams Awareness Month, which sees Citizens Advice and Trading Standards Services leading activities throughout the month of July.
Gumtree said it is working with various organisations to tackle online fraud and help keep people safe.
Scam victims were also asked what tactics had been used against them.
More than a quarter (27%) had been put under pressure to complete the transaction quickly, 17% had been encouraged to pay for the item without seeing it first and 15% had been persuaded to continue their discussion off the website they were using.
While 17% of those scammed said they thought they were getting a bargain, the same proportion also said they thought the advert was convincing.
Morten Heuing, general manager, Gumtree said: "This research has told us that users of online marketplaces are often the target of sophisticated fraudsters and scammers.
"We recognise our own responsibilities to help users stay alert and savvy to the tell-tale signs of fraudulent activity, but as an industry we must do more.
"Whilst millions of people use these websites safely and successfully, the reality is that fraudsters are out there exploiting honest users.
"Our Trust and Safety campaign is only the beginning of a long journey for industry alongside the one in three Brits who use Gumtree and other marketplaces. Only by recognising online fraud and tackling it together will we succeed in eradicating it."
Victims of scams in Gumtree's research said they had learned to physically see and test items out before making a purchase and avoid websites where they have had a bad experience.
They also said they would look for guarantees when making payments and pay more attention to consumer reviews and seller ratings.
The report included research among 2,000 people from across the UK and a further 1,000 scam victims.