I agree with working something out with the seller. Either s/he neglected to inform you of a pre-existing problem or neglected to pack properly, as there is no way that one should not anticipate the need to protect that section of frame when shipping. Frankly, I would not touch it until you have some compensation for the damage.

Now that I think of it I would advise you to have the dropout, rear triangle and hanger alignment professionally checked now. Checking all three can be done in about 15 minutes and can be done by any mechanic who has the right tools and knowledge. Of course you will be into more money if things need to be corrected. How you handle issues beyond the original one you posted depends on how the seller represented the frame.

If you give it a go yourself be very careful to put incrementally more bending pressure on it until it gives a little and then recheck. You don't want to get into the position of bending it back in. A longer lever used carefully is much, much safer than a shorter one.

p.s. One thing confuses me. It seems very odd if this was a whole bicycle rather than just a frame that the rear wheel not mounted for shipment.