Edward O'Hare, 46, who had admitted the robbery in an earlier hearing, was sentenced to six years at Exeter Crown Court.
The court heard that O'Hare had travelled to Honiton from London armed with cable ties and masking tape. On the morning of Saturday, March 22, 2014, he waited outside Banwell Antiques in Honiton until shop worker Sarah Richards arrived to open up.
O'Hare then bundled her inside, held his hand over her face before tying her up. He used the safe keys to escape with a hoard of 300 items of silver and jewellery with a retail value of £400,000. Richards, 58, has been unable to work since the incident and none of the jewellery has been recovered.
O'Hare, who had fled abroad, was caught when graphic CCTV footage of the incident was shown in a television appeal.
Sentencing O'Hare on September 10, Judge Francis Gilbert told him: "You subjected the victim to a lengthy ordeal. She was left shaken and shocked and was completely terrified and in utter fear she would be injured."
