From the other end, someone is accepting payment in advance i.e. selling a product that doesn't exist.
If you intend to purchase that product (or constituents of it) from third parties, then that becomes selling a product that you don't own (the third parties you intend to buy it from currently own it).
Now you can say it doesn't yet exist, or isn't yet bought, in order to fulfill the order - but the argument for selling stocks you don't own, is that you don't yet own them. I see no reason to say a genuine intent exist when fulfilling a purchase order, but not when returning a stock; kick-starter provides enough evidence against that..
How about if you sell a share that you bought within the last two days. I mean, your buy trade hasn't settled yet so you technically don't own it yet. Is that fraud?