The sender has no intent to provide the item or service and your money is lost.
Often, these scams offer a tempting discount or claim to have 'limited availability'.
Fraudsters typically ask you to send money via bank transfer.
Here’s the cautionary tale of how one HSBC customer lost almost £1,000 after falling victim to a purchase scam.
The details are based on a genuine case but some details have been changed to protect the anonymity of the people involved.
Tom has banked with HSBC for several years and was helping his father look for a van to help him move house and to use after the move.
They saw a 2018 VW Caddy van for sale on an auction website which they thought looked like a real bargain.
It was a business seller asking for a £950 deposit, with the full price advertised as £4,500.
Tom said: "It seemed ideal and much cheaper than I was expecting. We were very excited to have found such a good price, so we were eager to go through with it before it was sold to someone else."
In his excitement, he paid the deposit by bank transfer and drove from his home in south London to the address in Brighton given by the seller.
But when he spoke to the people living there, they told him there was no van for sale.
They were puzzled by what was happening because somebody else had also knocked on their door earlier to ask the same questions.
It was at this point that Tom realised he and his father had been scammed.
"I couldn't believe it. I was so angry that we'd been tricked in this way. The van I'd paid a £950 deposit for didn't even exist, he said.
Tom reported the fraud to us and we looked into what had happened.
The true market value for that type of van with 90,000 miles on the clock, with no damage, ought to have been closer to £14,000, not £4,500.
The price was too good to be true, which should have set alarm bells ringing.
Tom confirmed to us that he and his father didn't carry out any checks on the seller.
There was no discussion about the size of the deposit, which was 20% of the value of the vehicle.
Tom also didn't ask why a deposit was needed before he had chance to see the van.
There were other tell-tale signs that Tom should have been suspicious about - for example, the seller's bank account was a personal one, even though it was listed as a business on the auction site.
By the time Tom reported the scam to us, the money had gone.
"Later on, I felt foolish because we had clearly rushed into this without thinking straight," said Tom.
“I've learned my lesson the harsh way. I won't be mistaking the same mistake again, that's for sure."
In Tom's case, he should have completed checks to make sure the item and seller were genuine, such as:
Be suspicious if a seller wants you to make a bank transfer – or asks you to send money before receiving an item or service.
Remember, if an offer seems too good to be true, it probably is.