...buy some cartel packs to get items to put on gtn or website....
You're still missing the point. It's not just "some cartel packs", it will be a lot of them, as much as it's needed to spawn the rare items.
Let me give you an example with made up stats :
Let's say the rare Mask has a chance of 1 / 50 on every pack.
That means on average that for each player that gets a Mask, Bioware sells 50 packs.
However the player gets the Mask, directly from a pack he purchased from the Cartel Shop, or from the GTN using in-game credits, does not matter,
as for each Mask to exist in the game at first, about 50 packs will have been bought by other players (or by gold sellers as you suggested).
Bioware will always still sells as much as pack as normal as long as there is a demand for the Mask. Gold Sellers have no incidence on that aspect.
Gold Sellers do have a negative impact on the Cartel Market for another reason: Some players buy Cartel Market items for the sole purpose of reselling the items to other players as a way to quickly earn in-game credits.
And to earn lot of credits, they need to either be lucky or to buy lot of pack to find rare items.
But instead some of these players might be tempted to just purchase credits directly from Gold Sellers. That"s what hurts Bioware sales, because then the player money goes to Gold Seller not them.
One theorical way to prevent such problem, would be as you suggested to sell directly the Mask in the Cartel Market. But then Bioware would have to sell it to 50x the price of the pack to compensate (in that made up example) for the lack of profit made by gambling.
Then I can already hear someone saying, that they could lower the price, and that more people would buy it. Not really because if it becomes easy to get, it loses its rarity and becomes worthless
Also I guess having a heavy priced virtual item would have a negative psychological impact.
And somehow gambling is more fair, as even someone that cannot afford to buy lot of pack, can still get one if lucky. While if it was just sold at high price, only most wealthy people would get them from the Cartel Market.