This really is an interesting subject but I would forget about flashlights
being "investments". Before the WWW took such a foothold, things went way up in value but now with the saturation online of everything from Beanie Babies to rare coins, the internet has made the unobtainable, easily obtainable.
When CPF first came online, early lights from Mag, the genesis lights from Surefire and landmark companies like Tekna were highly sought after. With the advent of CPF and Ebay, over the years they have became commonplace. Heck, a rare Tekna dive light that would have been unobtainable just a couple of years ago, I recently purchased for $10.00!
Whatever is hot will sell and the "Larry Lights" and the "Katokichi Lights" will always have a place in our hearts here on CPF but I've seen great deals on both and have seen both sell for well under what they were first offered for. I have seen them go on auctions here on CPF for double or triple their original price but that was from very generous folks that wanted to support CPF.
The best investment lights I have ever seen, (and that was for an "eye-blink"), were the ARC's when ARC went out of business. The original ARC's sold for double or triple, (and more), of their original cost. Now that ARC is back in business, you can again pick up those old ARC's for a song.
Sure, there will always be certain lights that may become investments, but with the march of technology, what was once unobtainable at any price will soon be commonplace. As an example just look at the JIL lights. Tiny, bright, super quality and AFFORDABLE. The future of lights is extremely exciting but lights as investments has a ways to go.