Bought Macbook Air but cannot factory reset...

Hi there,

I bought a Macbook Air with Mountain Lion installed. Stupidly, the previous owner didn't remove many of her settings or data so I'm doing the factory reset that she should have done. However, she's clearly upgraded from Lion when she bought it.

The problem I have is that, when I try to do a factory reset, it tells me that my Apple ID did not purchase Mountain Lion and therefore refuses to let me factory reset. I can't buy ML off the App Store either because it says that it's already been installed!

What do I do?