As a buyer, I'd agree to pay to have the block magnafluxed and spec'ed out for a rebuild. I would require that the seller agree to give me back what I paid for the block if the block is not rebuildable.
If shipping is involved, I'd pay for shipping to me, and I'd expect the seller to pay shipping back to him/her if the block is no good (and, if the seller wants the useless block).
