It all depends. Plenty of people sell stuff they dont know how to fix.
Last year I bought a 6k zero turn (12k new) with 700 hours. Guy thought it needed a new engine because it was running strange. I figured out it needed a coil. (I even had a used coil on an old motor for basically free) Paid $1,500 for it and sold it for $5,500 after cutting quite a bit of grass with it. I miss that mower
While true, if it is dead, and the seller not able (or willing) to repair the price a buyer pays should be based on the worse case scenario. 'Just needs a head gasket', 'Just needs a carb cleaning', just needs a new spark plug', all to me mean it needs a complete engine rebuild or why else didn't the seller simply do it or have it done?
My wife could fix it, she'd call a dealer and have them come, get it, repair it and return it.
What you paid was reasonable being what was wrong was an unknown, his loss was your gain.
Damn Keurig machine stopped running the is morning....
