People buy from a proshop because they expect some sort of guarantee that the ball is o.k. Your reputation is worth something. A reasonable person would not expect that a used ball is going to crack like that. If you, as a proshop, do not intend to meet your customers reasonable expectations when they purchase a used ball from you, you should make it clear that you are selling with no warranty.
You failed to do this, so take some of the responsibility for it. But, I would not give the guy a new ball. I would give him a prorated credit, or, if it was cheaper, replace the used ball with a clear disclaimer for future reference.
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Those that can do. Those that can't complain. Pimpin ain't easy, but it's mandatory.
Most things we like, we don't need. Most things we need, we don't like. Don't confuse your likes with your needs.