The temp light will go off and on as it tries to maintain a steady temp. However, if the temperatures are that
far off what they should be, it is definitely either a controller problem or the pan is bad.

The seller should offer
a refund, or perhaps you have a friend with a controller you could borrow to see if that is where the problem is.
I would never buy a used or even factory-refurbished electrical device without a guaranteed return policy.

N.