So in this config, only the last line shows up in game. When I remove cobblestone, dirt one shows up. And when I remove dirt one, the emerald ones show up. Only the last line shows up no matter what. Is it because, more than one "1*thermalfoundation:coin:64" can't exist?

"custom entries": {
"__comment": "You can define additional items to be sold by the Market here. Defaults can be overridden. Prefix with ! to blacklist instead.",
"examplemod:example_item": "2minecraft:emerald",
"1thermalfoundation:coin:64": "1minecraft:emerald",
"1thermalfoundation:coin:64": "64minecraft:dirt:0",
"1thermalfoundation:coin:64": "64*minecraft:cobblestone"
}
}

I would recommend something like https://jsonlint.com/ to validate JSON files.
Yours for example misses some things.

Is it because, more than one "1*thermalfoundation:coin:64" can't exist?

Also you are not allowed to have keys with the same name.

If you're talking about the star signs next to the numbers, they don't always show up for some reason when I copy/paste them here.

Ah I see, I hope it's a feature in the future or something. It'd be nice to sell things.

#17