Vintage guitars are usually a good investment, especially the older USA-made Martins, Gibsons, and Fenders. I've played for years and have bought,traded and sold numerous guitars.
With the introduction of e-bay, everything changed. Used to be you could go into a music store,pawn shop, or through classified ads and pick up an older Fender or Gibson for a reasonable price, but now shop owners and private sellers have access to a world-wide market and that has really driven up the prices of vintage,collector-quality instruments.
I would suggest to anyone who has a vintage Martin, Gibson, or Fender to hold on to it unless you just really need the cash. Most everything made today is Mexican,Chinese,Korean, or made in India,
using poor quality materials and assembly line production by cheap labor workers, most of whom could care less about the quality.
Those old Martins/Gibsons/Fenders were made with old-growth timber and top-quality materials and most of the building/construction processes were performed by craftsmen who took great pride in their work.
In other words, they don't make 'em like that anymore!
So they will only appreciate in value.