But as far as I know, the coins issued by MLM firms are not traded in stock markets. Are there any MLM companies traded on stock exchanges?
You may be talking about MLM coin getting listed in altcoin trading platform other than their internal. The answer is no and even if there are any, the price on external exchanger will be way lower than in internal so you are forced to sell those coin on predetermined manipulated price in internal trading platform.
Better not to get fooled by all big promises made by those crypto based MLM projects.