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I would, if he claimed it was in good shape and it was not I'd be asking right out the gate, and be sure to give him good proof to in case he really didn't know so he/she dosen't think you are trying to screw them over. Just my two cents.
Ask yourself, was it functional as purchased and did the seller misrepresent that fact? You chose to tear it down and determine the exact amount of wear after the fact of purchase. Did the seller actually use the words "great shape" and how would you justify the conditions you found? Did you pay fair market value, or did you pay a premium for something in exceptional condition? Be honest here! Great shape does not actually specify a number, only functionality.
If I was the seller and you came to me with a suggestion like this, I likely would not be polite.
Well if the seller didn't tear apart the clutch pack, yet he said they're tight, then he probably just checked it out by hand or when it was still installed on the truck. Sounds like despite the worn clutches the S-spring was able to maintain enough pressure to allow the thing to work like it should, hence the seller's claim.
Gears look okay to me as well.
I wouldn't be asking for a refund... it's a used part and likely still worked as designed when tested.
There is alot of loose terminology getting thrown around here. I would not oblige to a refund if I was selling. Used parts are cheaper for a reason. You could have requested it be inspected and pictures with measurements before you bought, that is always an option, especially for high dollar parts. If you are that concerned I would have just bought new. Me, I always buy with the understanding that it may not be what is described and pay accordingly. Get it cheap enough and in it's functional form it fits my need, then I roll with it, otherwise I buy new.