There was no evidence that Hartley knew that there was anything wrong with the mower when he sold it. Both of them lost a 30-year friendship. This happens often in matters of principle. It doesn't seem worth it sometimes. Weigh this when considering whether you want to pursue a claim or settle one you have started.
Money is just a medium of exchange.
Property can be replaced. Problems with warranties show up in service contracts all the time. In service contracts, one person promises to pay money to another to perform a service. The person who is to perform the service promises to do so in exchange for the money. Kevin was a penny-pincher.