Animal Crossing: New Horizons already has a thriving online grey market, where sellers are offering items, recipes, and stacks of bells all for real cash.
New Horizons is the most online-friendly Animal Crossing game to date, and certain enterprising players are using those multiplayer features to sell their goods and services. I won't link them here for reasons I'll explain in a bit, but if you take a quick scroll through eBay you'll see in-game merchants offering to sell you rare delights like every flavor of Star Wand or Animal Crossing: New Horizons Golden Tools. The bell selling market is also quite competitive.
While it isn't possible to sell the items directly through eBay, merchants typically offer to either deliver their goods to your island or invite you to their island to pick up your order. There would be no in-game recourse if they tried to scam you out of your money. You could always leave them a bad eBay review and try to get their merchant account sanctioned, though.
The problem is that this is all a big violation of the first section of the Nintendo Switch End User License Agreement: "The Software is licensed, not sold, to you solely for your personal, noncommercial use on the Console. You may not publish, copy, modify, reverse engineer, lease, rent, decompile, disassemble, distribute, offer for sale, or create derivative works of any portion of the Software".
It's fine if you run a little online market where you sell your friends Golden Tools for bells. But if you do it for real money, you've already given Nintendo the right to shut you down. A Nintendo legal squad probably isn't going to kick down your door, put a bag over your head, and carry you away to Binding Arbitration unless you're a big-time seller - but they might ban your Switch Online account.
You don't need to hit up eBay to make progress in the first seasonal event, just check out our Animal Crossing: New Horizons Bunny Day guide.