Is buying PL from another website safe ?

Definitely not. Most of the platinum on those sites is stolen from hacked accounts / credit cards. In the case of the former the platinum will be traced back to you and your warframe account can be banned. Not trade-banned, banned-banned. In the case of the latter, the platinum will be removed from your account once the card owner gets their bank to chargeback.

Source: My brother got banned for this.