Wow that was quick. This is looking very promising. If putting it on the marketpalce doesn’t work then you might want to consider www.patreon.com or https://gumroad.com/
Both allow you to sell your stuff per project.
I dont know how you set everything up so this might not work but it might be beneficial to add the “Max stacked items” to the item blueprint or provide an override so that it can be specified per item or item type.