So you haven't had a letter from HMRC like the OP - just a request to add your NI number from ebay? In that case, all you need to do right now is add your NI number to your ebay account. Ebay will send seller's sales details to HMRC if the seller goes through the reporting threshold - they don't have a choice. If you don't add it, then ebay still send your info to HMRC, just without your number, but will also eventually restrict your selling until you add it. So you may as well just add it now.
As for whether you'll have to pay tax, nothing has changed in that regard. The same rules and thresholds apply as before. I haven't looked at your listings but I'm sure you know whether you are trading or not. If you're selling off your own possessions then there's no tax to pay. If you're selling items you bought or made to sell then you should have been declaring it if your annual turnover (not profit) reached £1,000. Whether any tax is due on your profits or not will depend on your own circumstances, such as other earnings using up your annual allowance for example.
Nothing has changed from this time last year, or the year before that, as to whether and what you should be declaring. All that has changed is that ebay now have to report your sales to HMRC if you pass the reporting thresholds.
