Sussex Police said the man called on an elderly and vulnerable couple at around 10pm on Monday, May 20.
He tricked his way inside, police said, and stole a number of valuable coins from the couple's personal collection.
He is described by police as white, of tanned complexion, in his 20s, 5ft 10ins tall and of slim build, with short, cropped and dark hair.
Anyone with information about the man or who has been a victim themselves is asked to report online or call 101 quoting serial 1475 of 20/05.