This is what happens when a company or person's business model is nothing more than reselling inventory from wholesale distributors. Often times these companies are piss poor managed and they don't wholesale purchase the item until a customer PAYS them for it. So, technically, they never had any in stock to begin with because they blew all their business' money on other things and they can't afford to stock anything for quick shipping.
So, they take your money and try and order one from one wholeseller, then the next, then the next... and then they suddenly can't find one at the price needed to turn a profit due to things like price hikes at the wholesale level or just common out of stock items not being available. Thus, they drag it out because chances are they've already spent the money received on something else.
These companies tend to have dozens and dozens of bad complaints because they're playing kamikaze economics.