>Alice owns 100 shares. Bob borrows 100 shares from Alice, then short sells them to Chuck. There are now 200 shares long available to buy back- 100 owned by Ali

>Alice owns 100 shares. Bob borrows 100 shares from Alice, then short sells them to Chuck. There are now 200 shares long available to buy back- 100 owned by Alice and 100 by Chuck. That is not quite true. If after Bob has borrowed and sold 100 shares to Chuck, and then David comes along and makes an offer to Alice to buy 100 shares, Alice can only sell her shares once they are recalled. Which means Bob now has to scramble to return those shares, specially if Chuck does not want to sell (or if Chuck has lent out those shares and they need to be recalled in turn).