For most buyers, a grand piano represents the largest and most expensive piece of furniture in their home. It is foolish to believe that such a purchase is not emotionally driven. I've known many people to have an affair with a piano made ugly from institutional life but would absolutely think better of it before bringing it home, whether for their own views or those of a spouse.

To the OP, a traditional cabinet in a common wood choice like mahogany is unlikely to hurt long term resale bets. I do, however, find the irony in the question as the OP is primarily looking among used pianos for those that are currently undervalued.

Today and for many years, grand pianos are most commonly black. Uprights have shifted strongly, previously dominated by wood finishes and now I think they are slightly behind ebony. I wonder if 20 years from now we'll look back and think, where are all the pretty woods? The cost is so little to gain so much.

Black pianos are beautiful, but I observe that many people are choosing to deny their first love, justifying it as an extra expense, perhaps wasteful, or placing future resale speculation over current satisfaction.

It all depends on your expectation of a piano. A piano is a piece of furniture for some, a status symbol for others, and an artistic tool for the rest. The finish importance is very, or much less important considering your reasons to consider such a purchase. I still wince at folks who buy expensive pianos for status. I realize that it is good for the industry, however, I wish folks who really yearned for a quality instrument could obtain one regardless of their financial ability.