Some items can only be bought by people with premium memberships, such as Builder's Club. However, unlike premium places, TBC and OBC have their own exclusive items as well. With the more expensive memberships, such as TBC and OBC, users with those memberships can purchase the premium content from the lesser memberships.
Most packages as well as a few hats and gears are premium items.
NBCs can wear any premium item they bought previously but can't buy any more.