@JY - Not quite correct in this case.
Seller sold frame as having no cracks (we do not know exactly how this was phrased) and buyer goes to collect, inspects and agrees no cracks pays money, takes bike.
Buyer gets bike home, knocks out press fit BB and has frame stripped for a repaint and sees crack in frame.
Where is the concrete evidence that the seller lied.
They may have lied, they may honestly described the frame not being aware of the defect or the frame may have been absolutely as described and it has been damaged when in the custody of the buyer.
As others have said have the buyer got home and seen the crack and contacted the seller straight away they would be in a stronger position but with the work they have carried out how can they prove that the damage was not caused by them?
Scenario - you sell some forks and advertise them as no scratches on the stanchions. You have cared for them well, you have never seen any scratches on the stanchions. You even have a good look around them before shipping them and nothing.
The buyer contacts you and says excuse me, these forks are scratched to buggery, marks all over the stanchions. You say eh, no not those forks. The buyer responds - definitely there mate. Noticed them when I stripped them down to service them.
What would your response be?