To buy low, and sell high you have to be very fast, and continuously check the market. It`s pretty risky, especially with BTC, but can be more possible with altcoins. If you carefully choose the right coins due to their potential, price and market volume.
But again it`s not such huge profit. Yes, you can be lucky if you`ll buy really right altcoin at the lowest price, and then it`ll rise very high, but it`s pretty rarely. About Telegram channels, it`s tricky, many scams.