Sadly not all primes are available all the time. Mag, Ember and Frost parts can't be obtained at the moment. Super-nice people most likely already gave their sets away, while it's an investment for the others, and they'll expect hard plat for it. If you are rich, totally go for it, you might even get a sharkie discount.
However, it was announced that the Void will blow up or something in a few updates. I think it was also confirmed that vaulted frames will be available again by some new system. I'd consider waiting for that change to see how it affects the market while pursuing some other goals.