You will need to use the Online Internet Recovery system, Hold down the Command+Option+r keys at startup until you see a Spinning Globe in the center of the screen, and reinstall Lion on your system. Then if you want to can Buy Mountain Lion from the Mac App Store under your own Apple ID and install it.
This happened because all of the downloaded version of OS X, If bought as an Upgrade from a pervious version of OS X, are tied to the Apple ID of the person that bought it. It is not tied to the machine.
The only version of OS X that is tied to the machine is the original version that came Pre-Installed on it when bought New. The seller should of wiped the drive and reinstalled Lion on it before they sold it to you. So yes they lied to you. But more then likely were not aware of this fact.