Answer:
My second is performed by my first. A thief by the marks of my whole might be caught. What am I?
Though I am only two words I have thousands of letters in me
I can be swallowed But I can also swallow you.
While you were folding a letter you got me. Usually you will do whatever it takes to avoid me but now you can't help but find me.
Weight in my belly trees on my back nails in my ribs feet I do lack.
Whoever made me don't want me. Whoever bought me don't need me. Whoever use me don't know me.