People are urged to be extra vigilant after a man was conned out of £20 in Lincoln.
The scam involved a man asking for directions to the A1 in a car park in Tritton Road.
He pretended to be so grateful to be given directions by giving the person a watch to say thank you.
The man then claimed that he had been robbed and his petrol tank was empty and asked for some money for fuel, giving the victim another watch in return.
The two watches were very cheap and not worth anything.
A Lincolnshire Trading Standards spokesman said: 'In the vast majority of cases, these people are scammers and the plea for money is a deception. "
"These people prey on the good nature of our residents and Lincolnshire Trading Standards advise anyone who is approached not to give cash to strangers.
"Always stop, think and be sceptical before handing over any money."