If you buy stuff solely for personal use and then sell it on (for a lower/same price) when you're done, it's not a business
But what if you sell your own stuff on eBay and the end price is higher than you paid? For example, I recently sold a piece of jewellery that I paid £50 for ten years ago for almost £200. It was an auction, so I had no idea how much it would end up selling for (I started the auction at £25).
In the past I've bought items (new and second hand), worn them a few times then sold them when having a clear out and occasionally they've sold for more than I paid.
However, I'm definitely not a trader, despite having made the occasional profit on personal used items. If I'm starting my auctions at very low prices and there ends up being a bidding war I have no control over what items sell for.
Also, what about people who sell a few high value personal items a year? For example it's not beyond the realms of possibility that someone might sell say a laptop, a musical instrument and a piece of jewellery in year and the total is over £1000. They're not trading and they're only selling stuff they've bought and used themselves.
I was going to have a massive clear out in the spring but I'm not sure I will now!