Why my dentist loves me (Cannondale sob story)

Doesn't look like cracks to me. I'd save sanding this and just send the frame back if you can, on the basis that it was not received as described since it has been damaged to the point where it is unsafe to ride.

^This, the feck off holes in what is pretty thin ally of a decent age is a heap of trouble just waiting to happen. It also devalues the frame massively even if it were safe so the bid price does not reflect the intrinsic value of the frame as described.
Effectively it is misrepresentation of goods which comes under the Sale of Goods Act.
So even if Ebay/paypal play up the law is on your side.
Ensure that when you approach the seller you mention that under the SoGA1969(As amended) that last bit in brackets is equally imprtant btw it is the sellers responsibility to pay for return postage costs on top of your refund for the frame & postage.
The law is pretty clear on this so don't take no bullshit about whatever ebay/paypal might or might not state..UK law takes precedence