hi, i know from previous experience that (providing you used paypal) you can make a not as described claim for up to 80 ish days from time of receipt, very few questions are asked its mostly up to the seller to prove that the item was listed correctly, so you have a very good chance of getting your money back.
the issue for me here is that if the guy has a reputable selling record as you say he was probably either unawares of the crack or it has somehow happened in transit (very unlikely i guess) either way after 6 weeks the moneys probably been spent on something else so it would be hard on the seller if this was the case.
I would be tempted to price up a repair as 5th said above and speak to the seller, tell him that you would rather not put a claim in and if he could pay for the cost of repairs the matter would be sorted you can then claim a partial refund through paypal-if he is uncooperative go ahead and claim the full amount.
just my take for what its worth