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Normally I'd run away from things like this, but it's selling for a cheap price (half price almost) so it's tempting.

A) Is it likely to damage tone very significantly?

B) Is it easily repaired? (and cheaply)

C) How would it hurt resale value if well repaired?

Once the back is cracked it will hinder the woods ability to transfer sound effectivley.

This type of repair to do it properly will require the back to be removed. All cracks repaired as needed, and the guitar will then need to be refinished.

It will effect the resale value to a degree no matter how well it is repaired as time goes on the repair can and more than likley be noticeable. If the guitar is refinished with Nitro, as the laquer shrinks into the wood, or the wood expands with added moisture it will cause the repair to be visable. The repair will still hold and the guitar should sound fine, but it could scare some buyers away.
What kind of guitar is it? and what model is it? It looks like a Martin D15 from what I can see in the photo. If that is what it is I would pass on it, if it were a D18 and the price was right it might be worth it. This is not an easy repair to do well and not have it be visable..................... Good Luck!!

Yep, it's the D15.

Just what I suspected........ I'll pass.