Because its not easy to win auctions and the chance of winning an item in auction that u really really want is very very unlikely. You usually end up winning something stupid like winter staff for 50k gralats, sell it for 75k tro and buy something u wanted. At least thats what I did.
It has been a long time since they auctioned brellas which is an example of my point.
Shells are the currency of tradable items.