So for context, it may help;
If your into crypto when the main reporting kicked in there they (exchanges mainly) told you via email that they were reporting activity £x by agreement with HMRC.
HMRC then send a letter saying, we have been told by XXXX that you have used their platform and have exceeded £x. (based on deposits iirc not sales)
You are required to advise us if Z applies to you. Z being a profit exceeding your potential allowance, or if you basically know you should have paid tax.
If you do not understand we suggest seeking the advice of a tax advisor to ensure you are compliant.
I would expect Ebay etc will end up along the same lines.
Give HMRC data for people who exceed an arbitrary amount no matter what they have sold. Say £1250.
HMRC will send a letter to all those people saying if your trading and its exceeded £1k you must report, if you have sold personal items you may be liable.
But in both cases if you are unsure seek tax advice.
That way HMRC undertake little work. They have warned you, so should it later be found you should have paid tax they will likely be able to ignore the statute limiting time as they will say you consciously didn't pay tax as opposed to unwittingly didn't pay tax.