Those are just a few examples of scam accusations made here on the forum related to Telegram. Some are straight forward one to one scams (i.e. send me xx BTC and I will give you yy BTC after a few days), whilst others are group Pump and Dump scams that will make you lose a fair share in benefit of the promoters.
The Forum is no guarantee at all either, although you can feel a bit safer if the account you trade with has a high trust value (Trust is a forum feature, see Marketplace trust). You need to be very wary at all times, and it should deem much safer using the exchanges and then keeping your crypto of a device such as Ledger Nano S or Trezor.