People who are purchasing guitars as "investments" are a very small, niche market. There are some other investments that may indeed offer a higher rate of return in a shorter period of time. (I haven't got the time or the nerves to become a day trader on the stock market.)

For me, I like to invest in what I love, and that happens to be older Martins. I happen to also be a player and appreciate a fine instrument "that speaks to me when I play it" and as far as I am concerned these are playable works of art...and no other brand of guitar rings my bell than an older seasoned Martin.

I know from my own experience and watching the market and even tendering the occasional offers from stores who know of my instruments, that 2 of my guitars, 1 being a 1950 D-28 and the other the '47 OOO-21 I mentioned previously, that I could flip them tomorrow (if I was insane enough to do so) and see at least a 30% return on both. One I've had for 3 years, the other I've had for 4 months. But I was fortunate that I acquired both, for less than what the current market value on them was at the time.

It's a game of patience; and letting the right one "find you" and having the bucks available to move on the right deal on a moment's notice. Timing is everything as well as investing in the desirable ones that you just happen to enjoy playing.