Vehicles are machines, they break down. That's why their value goes down over time, more likley you'll have to spend more to keep her running.
People now days dont understand "Buyer beware" they want everything to come with a warranty and return policy. I wouldnt loose to much sleep over the issue, my dad always told me when you buy (or in this case trade) someone elses used vehicle your buying thier problems. You may have some as yet undiagnosed problems on your new truck, and if you do thier now yours thats the way it works.