Selling online can easily put you on HMRC's radar.  You might think your eBay or Etsy shop as a harmless hobby, but if HMRC disagrees you could run into serious tax trouble.  In HMRC's eyes, if it walks like a business and quacks like a business, it's a business.  If you're making enough money, or making it regularly enough, you'll probably have to pay some tax on the income.  Even things like Airbnb rentals come with tax strings attached (you can read more about that here), so don't get caught out.