I can stop a sale, protect a boundary, end a request, or save a life. I am only two letters. What word am I?

I am first in end, last in place, and appear four times in experience. What am I?

At a border I may be crossed; in a queue I may be joined; in a script I may be delivered. Same word, three places. What am I?

A camera needs me before the picture is sharp; a meeting loses me when every phone starts buzzing. What am I?

In a debate I may be made, missed, sharpened, or conceded, though I have no metal tip. What am I?

A violin string needs enough of me to sing; a room after an argument can have too much of me, even in silence. What am I?

A bank studies me before lending, a writer wants me after a quotation, and a card can run out of me at the worst moment. What am I?

No hands are needed to break me, no box can hold me, and keeping me matters only after I have been tested. What am I?

A diver feels me in deep water, a student before an exam, and a tyre when it is inflated. Too much of me can crush. What am I?