I assume that in the golden era of true luxury and indulgence (as opposed to mass produced masquerading as rare) that a typical watch lover would buy very few pieces and keep them for decades.
The idea that any consumer product should retain a large proportion of it's purchase price in the medium term seems rather unlikely. For a start look at the retail and distributor margins,,,probably about 60-70% of retail, never forgetting government theft (sorry,,,tax. It's only fair!!). That means that the export (ex factory) price is unlikely to be more than 40% of retail.
Using that as a starting point how would you calculate a used price?
I think it may take a decade for the true value to show.
In the meantime we must pay for our pleasures!!
Kind regards,
Antony