You make a reasonable point as frame geometry between types of bike and even from different manufacturers differs so much , that getting accurate fit is often a personal preference thing. I find that my own preference on sizing means I am wary of anything over a 56, as I normally ride a 54 or 55. ( I have a buddy who is shorter than me and rides a bigger frame to get a good seat tube length, because his legs are longer than mine).
I, on the other hand am ape-shaped with longer body and arms than legs, so I am generally happier on the shorter seat tube bikes.
Good luck with the sale though.
