Electronic goods, jewelery, perfume and some luxury goods are generally cheaper than the rest of Europe.
However, be aware that many of the U.K chains such as Marks and Spencer charge 'Channel Islands' prices. In other words they add on about 10% for 'transport' costs so avoid these shops and generally speaking the clothes shops in St. Helier which are quite often more expensive than the U.K with a limited choice.
Jersey now has a sales tax of 3% on all items.
The main shopping streets in St Helier are the pedestrianized King and Queen Streets.