If you buy goods and sell them on eBay, then you are engaged in a trade and need to register with HM Revenue & Customs and to prepare accounts. You need to do the same if you set up a shop on eBay.
If you merely inherit some goods, or are merely clearing out your attic or garage, and you sell them on eBay as a one-off, then there is no trade, but you may need to report a capital gain on substantial items on your annual tax return.
We are able to prepare accounts for eBay sellers. We can process eBay Trader CSV files electronically, and we will give you a 25% discount if you supply sales information in CSV format.