A short sale doesn't have an expiration date.

And you don't get margin calls if you're not buying on margin.

The expriation date of a short sale is the day the interest and fees required to carry the transaction drives you to bankruptcy.

Depending on your solvency and leverage, that's either close by or eons away away but it is real.

A short sale doesn't have a limit to how much you can lose, either. I'm sure you get my point.

If you're sure the market has a stock overvalued, how can you lose?

Again, by running out of money before the market comes to its senses. I'm not sure what's confusing about this.

One of the first lessons to learn is to cut your losses early and let your profits ripe.