I thought you can. If you buy a gun from a dealer and it blows up in your face due to something that's not your fault, why can't you sue the dealer?
Because it wasn't the dealer's fault. If you buy a used product you almost always buy it as is. If you buy a used car and the wheel falls off can you sue the car dealer? No, you're SOL. There wouldn't be all these gun auction sites if they were liable for everything that could happen with these used guns - they would have been sued out of business a long time ago.