Very well he could be honest. But still clueless on what was going on. It's hard to be honest about something you don't know about. Bypass the honesty issue, you just want to know the chances of the motor pinging and beating up parts.
A motor like that needs to be paid attention to by a knowledgable person to make sure it maintains it's tune. I'd be curious of the seller is "tired" of the car. Or wants to move on to something newer or tamer. That would be more clues it was too much car/motor for him. And clues he couldn't keep it in tune.
Fact of the matter that current state of tune of the motor is doing some degree of damage. Sure it could be just a little and insignificant or it could be longer and more significant. You need to weigh that risk.
