Bransgore Wines owner Cornelis van Biemen, who also sells jewellery in his shop, said he noticed a gold and diamond cross had disappeared from the shop while he was being questioned by police and Trading Standards officers.
He said: "We are a wine shop, but we also sell jewellery. We are only in a small, friendly village so we just display it in the window."
Van Biemen said he thought the thief took the opportunity while he was talking to police and Trading Standards officers after the test purchase on July 27.
"There were at least six people in the shop at the time and I was working on my own," he said. He added: "A man had been acting suspiciously in the shop, walking around and bending down to look at things, then a younger man came in and bought something. The next thing, police and Trading Standards came in and said they were doing a test purchase and that I'd failed."
Van Biemen said the test purchase had been "distracting", but Sergeant Tim Adams of Hampshire police said the operation was carried out in accordance with national guidelines.
"We are committed to continuing the test purchasing programme and for operational reasons are unable to discuss details of individual premises or tactics used during the operation," Adams said.
A police spokesman confirmed that officers were investigating the theft at Bransgore Wines.