A distraction burglar stole a jar of £10 and £20 notes from an 84-year-woman.
The thief knocked on the door of the victim's Vauxhall Grove home, in west Hull, offering trinkets for sale.
He was invited inside and the woman bought three items, paying out of a jar of money.
The thief then left, but returned 10 minutes later, asking if he had left an ID card in the house.
He went into the living room, at which point there was a knock on the door. The woman went to answer, leaving the man alone.
A short time later the woman noticed her money jar was missing. The jar contained a large sum of cash in £10 and £20 notes.
The suspect is described as a white man in his 30s or 40s, with thick, black, brushed-back hair. He spoke with a local accent and was wearing dark clothes and carrying a black holdall containing the items he was selling. He was carrying a small pink card for identification, although the woman was unable to read it.
Police would like to speak to anyone who saw the man in the area or who may have been approached by him at their door.
The burglary happened on Tuesday (December 4).
Detective Inspector Nicki Miller said: "This man has clearly targeted this lady because she is elderly and vulnerable and he has used this to his advantage.
"We would advise householders against allowing callers into their homes unless they know them or have an appointment.
"People should not feel under pressure to purchase goods or services from callers. If they do wish to buy something they should ask the seller to stay outside and lock the door while they go and get money.
"Doors should be kept locked at all times as a matter of routine to prevent sneak-in burglaries, particularly at this time of year when people have presents stored in their homes.
"We would urge people to keep an eye out for elderly neighbours in case they are targeted by unscrupulous thieves and report any suspicious activity to police."
Witnesses are asked to call 101, referring to log 363 of Tuesday, December 4. Or call Crimestoppers on 0800 555 111.
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