Gain a lot of me, you'll be heavy. Earn a lot of me, you'll be rich

You do not want me to be permanent. But to avoid me is a mistake. You can let me help you. But precious time it will take.

Silky and soft we are, perfuming your lives. Take us to your love, but beware of our knives. What are we?

Before I was taken I was used to take flight, and my partner's purpose was to help hide from sight. Brought together our function has been made anew, and now any scribe would be happy to have us in their retinue.

You can find me in the darkness, But never in the light. I make laughter lethal, And agreement into sight. You can find me in the soil, But never underground. A bunch of snakes together, Their voices do astound.

One falls but never breaks; the other breaks but never falls. They are opposites. They cannot coexist, but neither would know where they end or begin without the other.

I sell invisible things. After I sell what I have, I still have it. I sell what everyone needs but often don't want. Who am I?