On ebay, latest model iPhones (4 through to 5 ) sell like wildfire, and at good prices too. Apple products in general sell very well, providing there is no damage.
Blackberrys, not so much. You can get a blackberry for around £20, but alot of them don't even sell!
General designer gear really. Not so much second hand clothes, there are too many items of clothing on ebay already so nothing really sells. Designer earrings and bags are good sellers.
limited edition items.
Nike blazers/airs and especially liberty London air max, they go for £300-£500.