2018 MARKET UPDATE: THE STARK TRUTH IS, IT'S ABOUT TO JUMP BACK UP
Some books will never truly go out of style. First big Silver Age appearances will always find new homes when they come to market.
That said, TOS #39 has been struggling in recent years. There are just some signs of regrouping, with uptick in prices on various grades.
Recent sales have shown improvement, and it will probably move back up the list next year. One to hold if you have not sold off yours yet.
2017 MARKET UPDATE:
This book has been in a bit of a slump in the last couple of years, but it's begun to heat up again. Not quite enough to move it up the list at all, but it's worth keeping an eye on this book. It might be about to break out of its holding pattern.
It's one of the more common Marvel mega-key issues of the 1960s, but seldom turns up in nicer shape than VG.
As 1960s Marvel Key issues go, ToS #39 is right up there with the best of them.
Not as common as Avengers #1 but not as scarce as Journey into Mystery #83, the huge success of the movie franchise starring Robert Downey, Jr. has made Iron Man a household name, and kept prices buoyant.
Don't cheap out with a 0.5 or 1.0 ragged copy of this book. Save up for a VG with a nice-looking front cover. Note that the sea green color is prone to fading; try to get an example with the green as pure as possible.
See more about Tales of Suspense #39 here.