There were rumours that HMRC would accept 10% of the actually tax liability of big time Ebayers if they made the first contact, but would be looking to cane those that didn't take that offer. The figures are absolutely enormous - when you wonder how you can get most things cheaper on ebay, this is often a big part of the reason why. If you watch the people you (as a private individual) buy from, their rating can go up by 100 in a single day - that's a lot of tax avoidance.
I buy on ebay, I'm not much of a seller, and to be honest even as a self employed person I wouldn't even think to declare my activities in my tax return, pretty much like I wouldn't declare the profit from selling my old tat at a car boot sale either.