From the Trafford centre probably jewellery. I wouldn't go as far as a spaceship but there's planes, helicopters and cars worth millions. What about if you could add 'buying a business' onto that list? The Coca-Cola Company is worth billions.
What an open ended question - Most expensive item in the world...... Possibly the biggest, most luxurious superyacht or the most flawless diamond.
Trafford centre - If I take my wife, she would probably find it!!
Looking at the list of stores in the Trafford Centre, it seems likely that the most expensive thing sold there will be an item of jewellery:
http://intu.co.uk/traffordcentre/stores#