It is definitely not a black and white area of tax. If you are buying (even secondhand) with a view to selling at a profit, and habitually do so, then you will become liable to tax, as does anyone carrying on any sort of trade (market stall, weekly car boot sales, shops, ebay), unless they are somehow exempt, or making losses etc.
Selling your own stuff doesn't usually attract tax for two reasons: a) usually you're selling at less than original price to you, and so there is no profit; and b) you are not carrying on a trade. It is the latter area which is the difficult one, as it when a trade starts is not clear cut, but defintely buying clothes in order to sell them on will be caught - the Inland Revenue will look at the frequency and size of the transactions - it won't matter than you even wore them once or twice - if there is a clear pattern, and you're making that much money you are likely to be taxable.
Ebay can't state a policy as it depends on the sellor. If a shop is selling on ebay (say John Lewis) then it will be taxable on the profits. If I sell my old baby clothes, then I won't pay tax. Sole traders fall in the middle, and you need pretty good records to show you're not actually making a profit.
Ebay is wonderful for the Inland Revenue who are starting to take an interest though - all those records of what has been sold and for what price, with Ebay having either bank details or other variable forms of ID (eg aol accounts) on most users...... I understand that they're swapping attention from car boot sales to this for that reason. Your contact told you that the Revenue can't show that you made a profit - unfortunately it is up to you to prove that you didn't - Ebay will show clear details of the sales, but you will have to show evidence supporting what you paid.
If the Inland Revenue have told you that you are not taxable on what you do, then make sure that you get it in writing.