Banks and large investors need to offload billions of dollars worth of assets. Orders of magnitude more than what the average amateur will generally be working with.

If an amateur can actually guess when the downturn starts (a really big if), they should not have much of a problem exiting. Even if a large investor can identify the downturn, if they try to exit quickly, they will probably just exacerbate the problem, without even being able to get all the way out themselves