Honestly i dont have any idea also, on what are the most expensive things sold for bitcoins. But why you should buy the things that is expensive if you can buy a lot of small things in all marketplaces, coz there are still many places you can buy the most important and luxurious ones. But it is good also to buy the things even it is expensive as long as it can make you happy like buying a house for 5750 BTC or ferrari 2200 BTC