their premium members buy at bottom and sell you at the top. Avoid these at all costs. Few other things to avoid are, lending platforms like bitconnect(RIP), davor etc, McAfee - avoid this joker at all cost he is an actual nutcase and I don't think he knows much about crypto projects or have any deep knowledge of any kind on crypto, if you've been following him and his reccommendations you know this already, avoid ICO's with no cap.
they are in the worst cases working like that,
in the most cases they simply manipulate the exchange price,
the market is sometimes very stupid because he starts to accept a higher price, later