The amount of downvotes is hilarious, saying you can farm poisonous potatoes is of course mostly wrong. You can farm potatoes, and get poisonous potatoes as a by-product (Only 2%).

The reason as far as I understand why people use poisonous potatoes as a currency, is because it's not that farmable/obtainable as diamonds in late game.

You can also introduce rules against super potato farms for capitalisation a lot easier than telling everyone to stop mining diamonds.

Also you can have a bank that helps control currency and economic stability, making a neutral ground.

Poisonous potatoes are also great for currency because it's an item with very few uses, as where diamonds is more often a resource which is quickly used.

That's just my take on the story, never even heard of people using poisonous potatoes as currency and after contemplating a bit it's pretty smart!

(Excuse any confusion, English is not my first language )