The sale was only good through the end of September and they don't stock the panels at Home Depot. All these companies such as Home Depot, Walmart, Sears, etc are now advertising products such as Solar Panels that they don't have in the stores and likely never will stock in the stores. It will ship from the supplier not from them.
This advertising everything possibly related to them works for the stores as they don't have to actually warehouse the products but it still drives traffic to their website where they advertise other products that are in stock. So even if the sales commission they earn on solar panels is not a lot the hook and lure benefit of getting people to their website and then into their physical store for those other sale items that were splashed all over their web page makes up for that. With the demise of news papers and all those subscribers to TV channels that don't run advertising they had to do something and this is one of those things they have done to get you to see their sales ads.
Not saying it was not a good price during the sale, just saying don't go to the store expecting to find the item in stock because it is not sold in the physical Home Depot stores. Maybe someday solar panels will be common in stores but not likely as home owners are usually installing big panel systems rather than smaller panels and portable solutions. But you can find some at the marine supply stores now and again.