bought a used macbook pro and trying to do a new reset

I'm not sure exactly what's the problem you face with and try to do, though Mountain Loin was deleted from Mac App Store when Yosemite released. Try to install Yosemite instead.

If you want to reset your macbook pro to its factory new, out of the box configuration there are a couple different ways to do that depending on exactly which MBP you have. Look under the Apple menu in Finder and select About this Mac. If your MBP shipped with Lion or later it can be restored using Internet Recovery OS X: About OS X Recovery - Apple Support Some earlier models can upgrade the firmware to use Internet Recovery Computers that can be upgraded to use OS X Internet Recovery - Apple Support If your MBP isn't one that can then you would have to restore from the original set of gray install discs which the seller should have provided.