Unless your MIL's living room is the size of a ballroom at Versailles, it must look tremendously cluttered. It wouldn't matter what those 500+ objects in her living room were--they're just clutter to someone else and a real turnoff to any potential buyer. Is there no real estate agent involved here? If there is, that would be the person to talk to your MIL about this, since I assume she (a) actually wants to sell the house and (b) loves those clowns. It'd be better if someone outside the family explained the situation to her.
There are a few circumstances that I can imagine that would make the presence of the clowns moot. (1) The house is a tear-down anyway and anyone who bought it would be razing it and building something else on the site. (2) The house is amazingly underpriced for the market and the neighborhood. (3) The house is an architectural gem, perhaps designed and built by Frank Lloyd Wright or another well-known architect. (4) The property the house is located on is worth a fortune and no one would really care about the house itself or what's inside.
One other possibility also occurred to me: your MIL may not actually want to sell the house, consciously or subconsciously. And, of course, since she's still living there (is she?) she doesn't want to dismantle the decor she's happy with.