I am not asking what happens if I short shares but if I lend to others my shares. If the broker cannot buy those shares back, I face the risk of losing them, don't I?
So I am wondering: does using those brokers come with the risk that I end up owning nothing if I pick a very successful stock?
The broker uses risk management, margin calls, market hedging, capital reserves and insurance coverage to ensure you always get your shares back. You're not really in any risk.
Capital markets are far bigger than you can imagine and unless you're holding millions of shares short (which is unlikely as a retail investor), it's not going to be an issue even with the most volatile penny stock.
In the miniscule chance that the shares are unable to be acquired for whatever reason, you would probably get reimbursed at the current market price, although I don't know the regulatory specifics around this and you should discuss the details with your specific broker.
So I am wondering: does using those brokers come with the risk that I end up owning nothing if I pick a very successful stock?