To restate what the article already says: dealerships, not General Motors, are conducting the sales. As a matter of fact, most states have franchise laws preventing automakers from selling directly to the public, forcing automakers to sell through a franchised dealers. GM is partnering with eBay in effort to provide their dealers with another avenue to sell cars. Why not? GM and their dealers need to move metal and eBay is looking for new opportunities given their recent decline.
I can almost see this as an evolution of the Saturn "no-haggle pricing" model. If you've already decided what you'd like to buy, why not turn to eBay and shop dealers for the best price? While this may not serve the dealer's best interest, competing against a multitude of others online, it's certainly a plus for prospective buyers. It simply provides a more convenient way of doing what customers already do: price shop.