That's the fine for submitting a late tax return. The fine for not registering as a self-employed trader can be a lot higher, especially if you get discovered by the tax-man rather than just confessing ignorance and registering late.
The issue isn't about profit as far as the tax man is concerned it's about trading, or buying with the intention of selling for a profit. They have some rather clever automated software crawling ebay pulling details of all powersellers and people with high feedback scores and comparing the registered details of the account holder with their tax records. Ebay is fully co-operating with the inland revenue on this and will hand over any records the taxman wants to see.
The accountancy world is already very aware of this and the first prosecutions are underway but it's likely to be a few months before any prosecutions are concluded and gain any publicity.
Sorry to be scary, but people should be aware of this if they intend to make money on ebay or amazon by buying items to sell for a profit. The keeping of accounts is a simple process and the tax office will help you fill in the forms, though an accountant may actually help more and save you money.
This topic really should be a sticky!