The only way that you are allowed to buy a PLEX using cash is directly from CCP through the account management page. PLEX's are in-game objects and, therefore, are subject to the same rules as every other in-game object. The EULA prohibits the purchase/sale of PLEX's the same as it prohibits the purchase/sale of ISK or any other in-game object.
You are permitted to purchase GTC's from anyone that you want, as those are out-of-game objects. You can then convert the GTC into a pair of PLEXs if you choose to do so. However, I would suggest purchasing GTC's from official channels
, because purchasing them through unofficial channels places you at a high risk of fraud. CCP can do nothing to protect you from out-of-game fraud. If you are scammed out-of-game, you will need to take that up with local law enforcement, and they will be able to do very little if the seller is located in a different country.