I would agree that sellers should be somewhat accommodating to buyers' interests in conducting due diligence, particularly around undertaking a thorough pre-buy. That is how I would behave if I were selling my plane. Even so, I would gladly provide a refundable damage deposit for the sort of in-depth pre-buy that I have in mind in order to bond the seller against any negligence my mechanic might bring.
However, the demand for a non-refundable deposit adds to a handful of other frustrating issues with the way this listing is being marketed. I admit that I would have walked already if I didn't have reasons to believe that this plane has been well cared for. (The list of mods also leaves little to add...other than a sleeping bag and some Mountain House

) Still, underneath the paint and a nicely updated interior might be a solid airframe that will give me years of service, or one turning to powder. The only way to know is to open it up a bit.
Thanks for your perspective, guys.