I am not in the market to buy or sell anything, I’m just curious to know what thing, if anything, is collectible. In years past, it was baseball cards, Beanie Babies… what is really popular now?
My son (now 10) was into Yu-Gi-Oh! cards for a while and these other toys that I can’t remember the name of that were like Star Wars chess pieces - each had a motion they could perform and you set them up and there was a game with rules you could put them through…that was very hot for a while.
My daughter (now 8) is into Littlest Pet Shop, American Girl Dolls and Polly Pockets - each have things than could be collected, but I don’t know if they have the same “business model” based on collecting that BB’s did back in the day…
Bakugon? My boy’s just getting over that; I could never fathom the attraction.
ETA: Hey, WordMan, I hear old guitars are kinda collectable. 
That’s exactly the wrong question. You want to collect something that is not popular at all, something few people would ever think of collecting. Then when it becomes valuable, you’ll have cornered the market. But you may have a very long wait.
Magic: The Gathering was still pretty popular last year when I regained some interest in playing it. I’m sure it’s still popular today and most likely in a few years too.
Guns.
No, seriously, there is some serious money involved in WWI/WWII military firearms. Authenticated, all-matching Sniper Rifles (not cobbled together from parts after The War) can be worth anything up to $10,000 to the right buyer.
It goes without saying all that stuff is way out of my league…
There’s a saying in the Military Firearm collecting community: Remember, you didn’t pay too much for it- you just bought it too early.
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Baseball cards are still collectible. The problem is that there are so many different sets out nowadays that it’s almost impossible to keep up with everything.
And if you want to collect cards worth serious money, you need to get everything professionally graded, a rather time consuming process.
Buy something you enjoy yourself, something you can imagine keeping around your house for the rest of your life. If you buy something that bores you, you won’t have any feel for what items will eventually increase in value. It won’t be long then before you grow tired of the collection and sell it off for less than you bought it for.