Increasing numbers of consumers are being caught out buying "phantom" goods at an average cost of more than £1,000, according to a charity.
Citizens Advice has reported a 17% increase in consumers complaining that they have bought high-value items like cars and flights online which have turned out not to exist.
It said people were more likely to lose money in phantom goods scams than any other fraud, ranging from fake doorstep sales to investment cons.
Phantom goods scams involve fraudsters advertising cut-price deals, most commonly on cars, flights and furniture, on social media sites like Facebook and Instagram and online marketplaces such as Gumtree and eBay.
Similar scams have involved a range of products from jewellery and cameras to musical instruments and driving lessons.
The scammers post fake customer reviews to give the impression they are a reputable trader.
Citizens Advice said one man paid £2,000 for car insurance he found on Instagram from a seller who had recommendations from other users, and only realised it was a scam when a promised email following his bank transaction never arrived.
Another woman agreed to buy a houseboat for sale on eBay for £5,000 and exchanged emails with the seller before she was sent a link to a fake PayPal site to make the payment. She has been unable to get her money back.
The charity said 555 cases of phantom goods were recorded between January and March this year compared with 495 over the same period last year, a rise of 17%.
It released the findings ahead of Scams Awareness Month in July, which in conjunction with trading standards encourages people to report and talk about scams.
Citizens Advice chief executive Gillian Guy said: "Scams can have a lasting financial and emotional impact on people's lives.
"With so many people shopping online to compare deals, scammers are using numerous tactics to target people with phantom goods. They are drawing people in with cut-price deals and then persuading people to buy items with phoney recommendations from customers.
"It's really important that people don't rush into buying an item when they spot a bargain, but take some time to make sure it's genuine first."
Consumer minister Margot James said: "Scams like these can be devastating for those affected. That's why this Government has been cracking down on these practices to make the internet a safe space for users.
"Last year National Trading Standards identified and suspended over 500 Twitter accounts and 140 websites linked to scams, leading to hundreds of arrests."
National Trading Standards chairman Lord Toby Harris said: "Criminals are able to set up multiple accounts to sell fake or non-existent products from almost anywhere in the world, concealing their real identity and contact details.
"There are a number of steps people can take to protect themselves, including conducting basic due diligence before buying goods. This includes checking reviews or references of the seller and also looking for signs such as whether the website domain begins with 'https'."
Gareth Shaw, money expert at Which?, said: "Sophisticated fraudsters are constantly finding new tactics to persuade us to part with money, one way is through fake or non-existent goods or services.
"If consumers pay for these phantom goods using a bank transfer, there is little chance of getting their cash back from the banks involved.
"We have been pressing the industry and Government to take firmer action to better protect consumers from bank transfer scams."