[stcredzero]

Because "nobody plays them anymore", the inheritors of dad's property will just put the guitar collection on ebay or donate them.

You already have this in instrument markets for traditional music. Trad musicians are minorities in the general culture, but their market demand for once-mainstream mass produced instruments of yesteryear can still support prices. (Paolo Soprani B,C button accordion, anyone?) Sure, you still have the occasional find, where someone sells something for 1/10th the price they could have gotten out of ignorance.

I wonder if everyone is going to be satisfied with making music on a laptop? I also wonder if playing a guitar is ever going to stop getting people laid? If you have both of these conditions, then guitars might go the way of the Flageolett. Historically, "outdated" instruments flooding markets at bargain prices have tended to give rise to new musical opportunities in underprivileged communities. Occasionally, they give rise to new genres.