Unless you’re a registered dealer, you’re not allowed inside most used-car auctions, where a couple hundred thousand vehicles trade hands every week. But starting next month, General Motors will offer ordinary car-buying Americans their first peek at its private stash, named the “Factory Pre-Owned Collection.”

For the first time, GM is making available to the public its auction inventory?specifically, the former rentals, GM company cars, and lease turn-ins that dealers bid for across the country. When the website launches, anyone can search for one of the more than 30,000 GM used cars nationwide, none of which are listed elsewhere since they’re all owned by GM and the rental companies. Think of the celebrity factor: You just might snag a Corvette that Mary Barra herself might have heel-toed in stilettos. Or a low-mileage stick-shift CTS-V wagon, a rare specimen built almost exclusively for auto journalists. Essentially, the Factory Pre-Owned Collection (call it FPO) widens your selection of certified pre-owned cars and lets your participating dealer act like a CarMax, since they can have a car shipped from any of GM’s 45 auction sites, and there’s zero obligation to buy. Unlike CarMax, the final price is negotiable, as is the shipping fee, but FPO is not a wholesale discount program. Once your dream Malibu?for example?arrives on the lot, it becomes part of that dealer’s inventory and will be priced at retail, just like every...