At the time of the transactions you were a member seller on ebay.

Your signed up to the criteria changes as new T&C apply to all platforms as time moves on, at that point you leave the service if you disagree. Any held funds were from being a seller, ebay & others have a legal duty now to provide the sellers data to HMRC regardless of the seller status private or business where the criteria you go over makes the change to report the data compulsory by ebay to HMRC.

Your issue is with HMRC (not they will take any notice of your complaint) where they dicate to ebay and them to you so ebay are compliant, you need to provide the NI number so when HMRC get your data they tie that up with your tax affairs their end.

If your not working or just PAYE you will have personal allowances. If your self employed likely youd best declare it, over your personal limits, even if you dont sell enough to make a profit.

Selling your own items not as a business you will not pay tax but you still to declare your NI number even if just to eliminate you from tax due as they will want to combine all your potential income figures, if you have any from other sources. You cant hide from HMRC selling anywhere online, maybe for now only by cash at a car boot!

Ignorance of tax laws is no defence, some being asked going back four years of ebay sales, could be a wake up call for those who have obviously been business sellers going by the feedback numbers gained, much more than 30 in a year. Private sellers as hidden business sales have been avoiding the distance selling rules for far too long where business sellers have to comply with far more and pay more in fees.

The day I started on ebay in 2001, already a business elsewhere there was just no question I could sell the same products as a private seller, and is an addition to other places anyway, both online and face to face.