re: How do you "borrow" someone's stock share and sell it?

Let's see if we can simplify...it's not always an easy concept.

You Sell 1000 shares Stock you don't have at $50 per share. so you have sold $50,000.00 worth of stock

Your plan is to wait till the stock goes down to $1 and buy 1000 shares so you can fulfil what you sold earlier. Costing you $1000.00

You make $49K on the deal.

In this deal instead of the stock going down it went up to $500. Now when the investor has to buy shares to cover what they sold...they're losing almost $500k.