Even if the seller is not “in” any scheme, this could be money-laundering in action: buy an asset in cash from a seller that won’t ask too many questions, then resell it for a little bit less - voilà, clean money without losing much of it.

If the seller is actually “in”, the price might actually include some sort of commission for other services of dubious nature. This happens all the time in sports and other “emotional” markets.