I bought a pre seasoned frying pan from Amazon, does it need to be Kashered?
Unless it has a reliable Hechsher, the skillet must be kashered before use.
The proper way to kasher new cast iron cookware is through hag’alah, which involves bringing a pot of water to a boil and then submerging the cookware into the water. [If one mistakenly uses cast iron cookware with a non-kosher item, in many cases libun gamur will be required].
Hag’alah for cast iron cookware is complicated by two matters – the size of the cookware and its weight/thickness – as follows. Some cast iron cookware is so large that it cannot easily be put into a pot of boiling water. In such cases it is worth remembering that hag’alas keilim is different than tevillas keilim in that one may kasher the utensil piecemeal and there is no need to submerge the entire pot in the hag’alah water at once. The other issue is that due to the weight and thickness of cast iron cookware, the metal will not get hot if one merely dips into the hag’alah water for a few seconds. Rather, the items should be put into the boiling water and left there for long enough that the water itself comes back to a boil, which indicates that the cast iron itself has become heated to hag’alah temperatures.
See further here.