Does the following dialog sound natural to a native speaker?

A store is about to close, a late customer walks into the store. The clerk is hurriedly getting ready to close the store and leave for home, then he sees the customer:
Clerk: Sorry, we're closed for the night.
Customer: I won't be but a minute.

By the highlighted part I implied that the customer won't take up much of the clerk's time.
Also, what do you say, if you find yourself in a similar situation?

Thanks !
It sounds natural enough, but "I won't be but a minute" doesn't necessarily imply an intent to purchase, so it strikes me as just a tiny bit odd. In that same situation I would make it clear to the clerk that a) I wanted to buy something (if you stay open another five minutes you can make money) and b) exactly what it was, in order to make the transaction as quick as possible.

Sorry, we're closed for the night.

All I need is a couple of blue books.

In these days of electronic everything, it seems possible that once the register is "closed" the clerk would be unable to sell anything else that day anyway, however politely one might ask. More and more retail outlets seem to be under the control of some far-off central headquarters. I remember shopping at a department store not too long ago and being told the thermostat was under the control of the main office two hundred miles away.
Thanks, Del
I see your point!