I'll disagree, if the hours went from almost 10,000 to 2,000 in one year, then walk away from it. Give RB a chance to look at the evidence and make their own decision, but I think I'd do it by email, not in person, I'm a non confrontational guy that way, stick to the facts. Then look at your state law and the UCC if they don't let you off, or they threaten to sue. They can write anything they want in their contract, it's only enforceable if it's legal in your state. And I don't think that would fly for one second if the facts are as you represent them. In reality their worst case is having to advertise the thing on their next sale with the correct hours, they're not likely to sue you, they're going to sell it again, probably with the new lower hours and hope the next guy takes it for a more realistic price.
Now if you bought an old tractor for $5 or $10K at an estate auction and came back when you found out the hour meter had been replaced a couple years ago, that would be ridiculous. RB is in the business of selling junk, and they are protected from most claims from disgruntled buyers by their contracts, but you're not claiming damages, you haven't paid for something fraudulent that they let slip through, and you're not going to.