It says: "Do you regularly use Facebook Marketplace to sell unwanted items?
"Please be aware of a new scam which has been reported to us recently - especially in the Flintshire and Wrexham areas.
"Victims have been advertising expensive items on Facebook Marketplace and the suspects make contact offering to purchase the item.
"The suspects arrive to collect the item claiming they cannot pay in cash and offer to make a bank transfer.
"The victim is shown that a transaction has taken place and they hand the item to the suspects who then leave.
"Later the victim discovers that the transaction was unsuccessful.
"Enquiries are ongoing in relation to these recent incidents, however we would advise sellers to be vigilant and use trusted payment systems such as Paypal, or check with your own bank that transactions have been completed.
"You can report any suspicious behaviour or activity to us via https://www.northwales.police.uk/contact/af/contact-us/
and https://en-gb.facebook.com/marketplace/learn-more/safety/