Hi Mavis,
You can get technical detailed information on Macs at: http://www.everymac.com and from that site, the specific information on the MacBook Pro7,1 is: http://www.everymac.com/systems/apple/macbook_pro/specs/macbook-pro-core-2-duo-2 .66-aluminum-13-mid-2010-unibody-specs.html for the 2.66 GHz version.
You see varying degreees of quality in people on eBay...some are really good, some less than good. With a computer there is no real way to enforce the manufacturer's license requirements so you end up in the situation you have, a machine with an operating system that is not the original and all of the ensuing problems.
If you cannot get the original Mac OS X disks from the seller, you can get them from Apple, for a small fee. Just call Apple, I believe the UK numbers are 0800 048 0408 or 0871 508 4400, and provide the machine serial number and they will send a replacement set of machine-specific disks. The serial number is easy to get: click the apple at the left side of the menu bar, About This Mac and under the logo is the Mac OS X version, click on the version and you get the Mac OS X build number, click again and get the machine serial number, which is also listed where you found the MacBook Pro7,1.
As some of the others have said, use the Mac OS X installation disks for the machine to erase the hard drive and install that Mac OS X, then you can go to the App Store and purchase/download Mountain Lion if you would like.
To erase and install, insert the first install disk in the optical drive, restart while holding the C key (does not have to be the capital C) and after it has rebooted, choose Utilities from the menu, then Disk Utility. then select the hard drive and erase/format.
Next choose to install Mac OS X.
When you are finished you have a factory-fresh computer that you can setup with your own admin userID and password and standard users.
If you then go to the App Store and purchase Mountain Lion, do the download but DO NOT let it install immediately. Go to the Applications folder and there will be the InstallESD file, make a copy of that file somewhere else, such as your Documents folder...the final step of the installation is that the installer deletes itself. If you wan to make a flash drive installer as we discussed initially in this thread you will need a copy of the installer.
After making a safe copy, now you can click on the installer and let it do its thing. When ever you feel like it, you can then make a flash drive installer from that copy of the installer that you made.
Now you have the machine working just the way you want and it is truly yours in every way 😀