If the buyer was unaware of the crack on purchase, then regardless off discount, or if the seller believes it won't effect the play, he should be offered a refund or a replacement.
It is a damaged product, and the advice of "see how it progresses with use" smell off lekka esque customer services.
Bats take plenty of preparation and time to get into there use. Oil, knocking in, nets to get used to a new blade. If it "progresses" to breaking, then that's wasted time into a season you can't get back.
If it breaks in 6weeks time, will the replacement bat come, oiled, knocked in, and with 6 weeks worth of free nets and games to get it to the same stage as the original? I think not.
This is a non argument. Replace or refund.