In order to sell cards in public or private do you have to buy a pack of cards with credits or can you buy the 1000 clintz?
You must buy some credits, not just a pack.
But then your account will be unlocked forever.
What do you mean "unlock"?
The selling of card in public or private is available only after a purchase of credits (as a mean to stop multi accounts abuse).
See Game Help on the left side. Buying cards from other players and everything else is available immediatly.