You don't have to pay for it. You can go to YouTube and search "Construct 3 inventory tutorial" and they'll walk you through making a more complexed inventory system for free.
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I feel like whatever I respond with then the next reply will be asking me to make it.
Isn't that to be expected since many Construct 3 users are visual learners? Visual editors attract those types of people. I'm one of them. Unless the answer is simple, a complicated written answer isn't enough. Visual learners will most likely need to see an example to learn from it.
Adic.48
Here's an example. I didn't create it and I'm not sure who did. I think I got it from someone being helpful in this forum though. I think you can change it to fit your needs.
https://www.dropbox.com/scl/fi/y8satdsk7szbxeo7c6r7b/click_inventory_example.capx?rlkey=559wy1vrd488ohi1i8zbsg00e&st=atk2fp9u&dl=0