Passive voice (to be bought) implies "bought by any person" while active voice implies "a particular person who wants to buy it." In the second sentence, "too expensive to be bought" may also imply that the house could be given away, or won, or traded for something extremely valuable. Examples to further clarify: to buy = "The house was too expensive for my sister to buy." versus to be bought =
From the natural point of view, these two sentences, all though considered correct grammatically, are the something a native speaker would utter. When we say "something is too expensive", the meaning of buying is implied. For example: You are looking for a used car about 3-5 years old and whiling browsing through a used car lot, a "used car" salesman tried to sell you a 5 year-old BMW 3 series
Grammarfreak is correct: simple is better. "The house is too expensive." is enough. You don't really need the rest unless you want to clarify who is buying or how the house is being acquired.
Sorry about the mental error. Let me say it again.
From the natural point of view, these two sentences, all though considered correct grammatically, are [ not ] something a native speaker would utter. When we say "something is too expensive", the meaning of buying is implied.