I have the solution to that: Don't buy anything you don't plan on using, or plan on reselling. That way the market value doesn't matter at all to you.
For instance, I own a few custom lights. My two McLuxes and a Lunasol are probably the best in terms of holding their value since they are easily upgradable and considered highly desirable. But over the long term, once most users have forgotten what the McLux III PD is, and what the Lunasol 27 is, they will probably not be able to command as high a price. How many here remember what the SFIII stands for? FFIII? What's a LionCub? Arc LS (not Arc6)?
Also consider the fact that I bought each of them to use and carry (which I do, every day one of them is in my pockets), which will leave marks from carry and (ab)use, and this causes them to lose value the most. A light that is not perfectly mint (and it is very easy to leave marks on a light) will not attain full resale value, since many prefer their lights pristine. (I prefer my lights used and enjoyed.) If you carry your $100+ light it will definitely lose value, but if you liked it when you bought it hopefully you will still like it now.
One McLux I have owned and carried every day for three years and I still love it. It is like an old friend and I could not imagine selling it even if I didn't plan on using it any more. I could probably sell it for what I paid despite the wear marks just because McLuxes are so highly desirable but a mint McLux would sell for much more. I value many of my lights much more than they would fetch if I sold them because I enjoy using them so much so I just do not sell very much.
Generally speaking, the only flashlights that really go up in value are either limited runs or customs. But customs only go up if they are considered valuable, and they still seem to depreciate at some point, usually after the next "big thing" happens, ie the switch from Luxeon to Cree.
On the other hand, in the short term you are very likely to be able to make most of your money back if you decide you don't want the light anymore. How short a term depends on the perceived value and quality of the light. Generally speaking "trend of the month" flashlights where a new design comes out every month or every few months are hardly worth reselling and high end production, limited productions, one-offs and customs will see the best resale value. On average I'd think you'd break even unless you knew exactly which ones to pick up and when to resell.
Hopefully you will find a light you really enjoy and hang onto it for the next couple years and if you decide to upgrade or replace it, maybe you will even feel you've had your money's worth. For many of the older lights in my humble collection of users, I feel I've had just that.