Charging port

iPhone Service Pricing - Apple Support

Then make an appointment at the genius bar of your local Apple Store. They will explain your options. Or return the phone and get your money back if the seller did not disclose that it wasn't working.

Probably an issue with the logic board. (Assuming charger is ok and there is no dust in the port) It is not something they will fix, they will do a whole device replacement for $299 instead. I would rather get a new phone if I were you.

often list and dirt in the port can cause that and cleaning it out with a needle fix it