I appreciate that you asked this question. So many people don't think about this for so many things, not just guns. I'm sure many of you have other collections as well. I subscribe to minimizing the collection to keeping a minimal amount of things. In the past I used to like to collect things but should anything happen, natural disaster or random event it can do a lot of damage and the true value is never realized.
I like to keep things in a living trust that I update annually and cycle out things as I lose interest in them. This is one of the reasons why the marketplace has as many features as it does. These are all tools to help you move things to others with the same interest and get the best value for minimal loss. That's not always the case because you might have something rare but for those, I usually have a shop noted down, Where and how to sell.
Another member mentioned keeping a spreadsheet that could also have a rough estimate of value.
Big thing for you is things that appreciate vs depreciate. Firearms don't really go down. M4 will be out of style in a few years and you'll end up sinking more money into it. When I was 29, I bought a e60 M5 BMW, had a ton of fun taking it 150+ mph on the 10 at night. Then I blew up the clutch, flywheel, and gearbox by pressing the "M" Man button a bit too often. The bill was $20k to repair, but fortunately covered by warranty and only paid for the clutch $6k, tires another $1.5k lol, maintenance is also $600-$1k..
If I bought bitcoin with that money, this site would not exist and I'd have a yacht in a foreign country somewhere.
At your age, accumulation is the smart thing to do, but I wasn't smart at that time or maybe now for that matter.
I went back to Honda/Tesla, haven't had a problem since.