I will once again provide the link that will answe
Post# of 96879
Now you are correct that your broker is ultimetly responsible to you. That being said your broker may have paid a fee to someone like Goldman Sachs to borrow the shares they needed to fill your order. It gets tricky here because several other firms may well have paid to borrow the same exact shares.
This allows them to plead ignorance. The good news is T+0 is close at hand and when implemented eliminates naked shorting. You'll be hearing a lot about T+0 in the very near future but it is important to know Patrick Byrne of Overstock is the driving force behind this and it does have SEC approval.
It is going to change the way business is done in our markets and make the playing field much more level. Go NTEK!!!