Many years ago, a fellow student in my year came back from a trip to Hong Kong with a very expensive SLR that he had bought there. He had just walked through the green channel with it.
He continued to take it on trips overseas, always walking through the green channel.
The camera had been through the green channel four or five times before he was stopped. Customs knew from the serial numbers that the camera had been bought in Hong Kong and asked for the receipts for the camera and customs duty.
They confiscated the camera and he had to pay duty on the original cost, plus VAT, plus I think a penalty charge to get it back..
Andrew