Your question is a bit unclear, but it sounds like you want to sue the builder for a drop in home values,which you attribute to the problem with the retaining wall.
Unfortunately, houses in virtually all parts of the country have lost value over the last couple of years, some areas more than others. So, unless the landslide destroyed all those the homes, I suspect that it would be difficult to prove that an entire neighborhood of homes had their value reduced to the land value because of a retaining wall being built without a permit.
As for the realtors, their job is to sell property - if it doesn't sell, they make no commissions. Therefore they are motivated to find the price at which a property will sell, even if it means lowering the price below what the owner thinks it's worth. The owners that sold for the reduced price agreed to that price, in order to sell the property. There is no way to stop willing buyers and sellers - or their realtors - from doing a transaction they all want, even if it means that the market will know that your house isn't worth what it once was.
There are associations that handle this type of problems, and in fact, one of them is handling it for the residents of the community. You may want to ask around. Good luck and God bless you.