Not only is it what is sold, but surely, whether it makes any profit or Capital Gains? Last year, I sold around £4.5k of coins on ebay, but these were all as a result of upgrades to my collection, so no business transactions. Overall, I pretty much sold them for around what I paid, so from a 'business' point of view, there was no income or gain, as such, so how can HMRC determine what constitutes a business. On a completely different area, I have just about finished clearing out a whole load of my wife's unwanted clothing - about 8 charity bags in total. Looking at ebay, I could, if I was motivated, have put them on there, and made a thousand or so, overall. That is peanuts compared to what she paid, so no profit there, but again, how can HMRC determine what has been sold, and whether it is likely to have generated a profit?
Based on the above, it strikes me that HMRC will spend more time and money investigating and checking than will be gained in income to the Treasury.