I have sold quite a lot on eBay since covid, a few things trading but mostly items that i no longer need to clear the years of clutter from different hobbies.
I keep a list of the items I list that are purchased to resell.
eBay allow you to export data including the payments from eBay (ie after their fees). I regular export between dates and go through it and mark it in a spreadsheet whether each transaction was trading or not. I ensure I keep under the £1000 turnover limit.
For items that are not trading I put a small comment as to why it isn’t
I haven’t been asked for the information before, however I expected to be questioned at some point.
It doesn’t take too long to do.
In terms of whether they believe me, when selling my own personal used items I very rarely sell more than one of the same thing.