From [0] and [1] I don't think chaining shorts, as described in the parent comment, is possible. Alice cedes control over the shares in return of a fee. Therefore, when Bob borrows 100 shares from Alice and sells them to Chuck, there are still only 100 on the market, not 200.
> When a security is sold, the seller is contractually obliged to deliver it to the buyer. If a seller sells a security short without owning it first, the seller must borrow the security from a third party to fulfill its obligation.
To actually answer your question, is worth noting that Bob has to provide a collateral to Alice at 100% + some margin (from wiki) the value of the borrowed stock. So, in short (ha!), Alice sells Bob a bunch of shares with the idea that they will revert the transaction at a later date.
> When a security is sold, the seller is contractually obliged to deliver it to the buyer. If a seller sells a security short without owning it first, the seller must borrow the security from a third party to fulfill its obligation.
To actually answer your question, is worth noting that Bob has to provide a collateral to Alice at 100% + some margin (from wiki) the value of the borrowed stock. So, in short (ha!), Alice sells Bob a bunch of shares with the idea that they will revert the transaction at a later date.
[0] https://en.wikipedia.org/wiki/Short_(finance)
[1] https://en.wikipedia.org/wiki/Securities_lending