That requires complete PVM balance between professions, which is impossible without further upsetting pvp balance, and well, just plain and simply impossible.
I have suffered because of this phenomenon in the past, and always blamed the players, not the designers of the game. Professions that are completely shunned in pvm, such as traders, mps and fixers are perfectly fine and useful for a raid, people just dont let them in because they want to minimize the time the raid takes and the amount people to divide the loot with.
S42 and Tara are the only remaining raids that cannot be done by a small group of people so every profession is welcome to participate . Strength is in numbers and as the risk of failing a raid outweighs the greediness of the raidgroup's members. And people are constantly bitching about these "waaah waaah it takes too long to get phats waaaah"
As for cred selling... it cant be eliminated. Live with it.