I'd keep the pistol, but I'd let the seller know I wasn't happy with the description. Might ask for a partial refund to get the damage repaired and see what happens.
As for the rust, it may be from blueing salts retained in the metal (silver solder???) used to attach the sight blade to the barrel. I'd ask a gunsmith's opinion. If it is from blueing salts they have to be neutralized or it will continue.
If a gunsmith thinks that is a possibility I'd contact the manufacturer.