Answer:
I have a straight back and sharp teeth to cut objects.
I don't live within a house nor do I live without. Most will use me when they come in and again when they go out.
I begin and have no end. Eventually I will be the end of all that has begun.
I make you weak at the worst of all times. I keep you safe, I keep you fine. I make your hands sweat and your heart grow cold. I visit the weak, but seldom the bold. What am I?
I make a living following the wealthy and beautiful.
It has no top or bottom, but it can hold flesh, bones, and blood all at the same time.