Understanding the argument -
The conclusion is, in a way, saying that with falling demand, one would expect 50 houses out of 100 to get sold, but the drop is even sharper. Only 10 out of 100 may get sold. So it means that there is something else that is causing this decline. That is what option E does.
Option elimination -
(A) A drop in demand for houses depresses prices disproportionately if earlier strong demand stimulated much new construction. It describes a scenario that's not in the scope of the argument. Out of scope.
(B) Real estate agents do not earn the commission on houses they help sell until the sale is actually executed. When the agents get a commission, it doesn't explain the result. Out of scope.
(C) Many people sell their old house below the asking price in order to obtain the funds to buy a new one - This may increase the number of houses sold. And doesn't help explain the result. It's not giving us any alternate reason that makes the drop sharper. Opposite of what we are looking for.
(D) In a housing market with rising demand, even houses that were originally overpriced usually sell at the asking price. - We are looking for falling demand. Out of scope.
(E) For many sellers of houses, the initial asking price comes to represent the true value, below which they will not sell. - perfect. This gives us an alternate reason. That reason is the high asking price which will add fuel to the fire and make the drop sharper.