HOW to reinstall OS X Mountainlion-original owner-"this is temporarily unavailable please try again later"

The 2010 model originally came with a special version of 10.6.3 or 10.6.4. The retail copy of the 10.6.3 Snow Leopard DVD will not work because if is a different build. You need the original DVDs that came with the computer when it was new. It did have disks. It does not support Network/Internet Recovery. All you could do is reinstall whatever version of macOS was installed before you erased the drive, and only if you did not end up erasing the Recovery HD that may have existed.

The computer you have came to you used. You are not even the original owner of the macOS that was installed meaning you could not legally upgrade macOS. It also means that the person who sold it to you did not provide the Software Restore disks. Unfortunately, Apple likely has no copies of them to sell to you as replacements. You would need to try a find them on eBay, Amazon, etc. I can only suggest you try this:

Install Lion Through Yosemite from Scratch

1. Restart the computer and after the chime press and hold down the Command and R keys until the Apple logo appears. Wait for the Utility Menu to appear.
2. Select Disk Utility from the main menu and click on the Continue button.
3. After Disk Utility loads select the volume (this is the indented entry, usually Macintosh HD) from the side list. Click on the Erase tab in Disk Utility's main window.
4. Set the Format type to Mac OS Extended (Journaled.) Click on the Apply button
5. When the process has completed quit Disk Utility, and return to the Utility Menu.
6. SelectReinstall OS X and click on the Continue button.

This will install the version of OS X you had installed.

The above will work up to a point. If what you had already installed was not obtained using your Apple ID, then when the server checks for an Apple ID it will require you to enter the ID of the original owner. Without it, you will not be permitted to re-download macOS.

How To Do A Factory Reset

Selection A should be used on computers that came with Lion or later when factory new. These models had no disks included when new. Selection B is for Macs that came originally with Snow Leopard or earlier. These models shipped with Software Restore disks when new.

A. Factory reset of your Mac - Apple Support

B. Factory Reset Your Pre-Lion Mac

Follow these instructions until you get to Step 5: Factory reset of your Mac - Apple Support. At Step 5 you will need a Snow Leopard DVD or the installer disc that came with the computer.

1. Boot the computer using the Snow Leopard Installer Disc or the Disc 1 that came with your computer. Insert the disc into the optical drive and restart the computer. After the chime press and hold down the "C" key. Release the key when you see a small spinning gear appear below the dark gray Apple logo.
2. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Utilities' menu. After Disk Utility loads select the hard drive entry from the left side list (un-dented entry - mfgr.'s ID and drive size.) Click on the Partition tab in the Disk Utility main window. Set the number of partitions to one (1) from the Partitions drop down menu, click on Options button and select GUID, click on OK, then set the format type to MacOS Extended (Journaled), then click on the Apply button.
3. When the formatting has finished quit Disk Utility. Proceed with the OS X installation and follow the directions included with the installer.
4. If you are planning to sell or give your computer away, then do the following: After you reformat your hard drive and reinstall OS X, the computer restarts to a Welcome screen and asks you to choose a country or region. If you want to leave the Mac in an out-of-box state, don't continue with the setup of your system. Instead, press Command-Q to shut down the Mac. When the new owner turns on the Mac, the Setup Assistant will guide them through the setup process.