It's your word against theirs really isn't it?
You can quite easily answer three or four simple questions at the Gov site here and come up with the answer that you have nothing to worry about.
So, you sell records on ebay fairly regularly:
You could argue these are part of a personal collection built up over many years which you are now downsizing (they are not part of a set as such, but they are personal belongings). So no problem.
On the other hand HMRC could argue that a pattern of fairly regular sales over an extended period of time indicates you are trading. So problem.
In reality you are doing a bit of both - selling items out of a collection built up over many years, some of which has been built up by buying cheap at car boots, chazzas and the like, and some of those purchases made with the express intention of flipping for a profit (hopefully).
So it's a grey area, and many accountants websites, such as this one, don't really help clarify things. In the end where does the burden of proof lie?