Why would someone tie string around the temperature gauge

I recently bought a car from a private seller and I know I got taken.
So today I was driving the car and noticed that the air conditioner would be freezer cold in idle or park, but while driving it would get unplugged refrigerator cool. This led me to check the oil, coolant, and transmission fluid, All of which were fine. I started looking on the dash and noticed there was a string tied to the temperature gauge holding it just below the half way mark. Common sense tells me the car may have a overheating problem or worse. So they tied the temperature gauge down to stop it from showing just how hot the engine is really getting to a buyer, me in this case. Also I noticed the fans wont stop running.

Does tampering with a gauge of that importance void the sale legally, kind of like odometer fraud?
Also does anyone have the slightest idea as to what could be wrong with the car so I can minimize the cost?
Thank you to everyone who decides to help me.