Technically Prime items cycle out of the relics after a set time...Or as Warframe calls it, they get vaulted. Prime items cannot be purchased from the marketplace leaving them only obtainable from purchasing/already owning Platinum and buying them from other players. (Or getting old relics popped on your runs, big luck required though)
So..technically wrong on that one.
Is the currency used to purchase said items obtained only by purchasing it? As far as I'm aware, the Prime items get vaulted as to not dilute the loot pool with too much relics, whereas you are literally getting limited on your option of missions and activities. It's as if Warframe's new disruption game mode that was recently released will be going away 3 months from now. Getting both Platinum and the needed drops is much easier compared to Destiny, however.
Platinum can only exist in the game through MTX; no other sources.
It's kinda a weird case; you CAN go through the game without ever purchasing it directly yourself by selling items in demand for it but this is only because someone else out there bought the Platinum you just traded for.