Ah you see, it's all about how much your opponent values the marbles. Marbles might be pretty dead, but there have been a hundred other trading games, with more or less the same conceit of some contest for acquisition of the opponents item. Marbles didn't have strong enough marketing (by who, the British Marbles Board?) to beat out the more modern crazes that laser-focused on kid's psyches. I don't know when they started doing that, but at least by the 90s which were rife with branded crazes that absolutely short-circuited young brains, and it's continued to now.

I remember football stickers bring banned on day 1 of term, having blown up over summer because someone stole a huge stack from a locker. Presumably the 90s ish was when the Made in Taiwan plastic crap availability really started to make that stuff cheap and fast enough to churn out in huge volumes to start a craze in weeks. Compared to marbles where a collection might represent years of growth, overall marble-econony production being trickled in by kids buying or being gifted just a handful at a time.

The actual games played with marbles is pretty varied. The local one that we had would have you put a marble a bit in front of your ankles, with your heels making a 90 degree angle. Then the other player would shoot at it from some distance away.

That variant was also a gambling type where you could win or lose. The shooting marble was often a metal ball, but the ones you wagered were the nice ones that everyone was after.