Everyone would agree the 30 items per year is low, but this is what has been set, 30 items or £1,700 in sales and eBay send the details of those accounts to HMRC.
You are selling second hand , used, personal items, you do not fall into the bracket of a trader, that is, you're not buying items to sell, or making items to sell, or selling all Brand New items, so this is not a tax you will need to be paying.