As the popularity of ACNH grew, so did online third-party websites that help make your island management more efficient. Nookazon (not affiliated with Nintendo) reigns supreme when it comes to buying and selling items to other real-life players. Basically, this is the eBay of Animal Crossing.
The premise is simple: A player can create a Nookazon account and list their Spooky Tree decor, for example, at 200,000 bells. This price is determined by the seller and can be referenced against others selling the same item. Once a buyer is interested, a temporary code for the seller's island is shared, so the two can exchange said bells and Spooky Tree.
Over time, as more sellers and buyers have joined Nookazon, items such as our Halloween-themed example become priced higher than the everyday "common" items. Yes, we’re talking about supply and demand.
And that, folks, is an entire economy.