i have been trading part time for a couple of years, not so much buying and selling more servicing.
insurance isn't that harsh, i've never been asked to prove one way or another, even after an accident. some have mentioned keeping hold of receipts for parts, but even then ebay doesn't give receipts as such as there is no vat anyhow.
i wouldn't worry about the past, concentrate on the future, if you make money on something then declare it anyhow, you may get away with it for a while, but eventually someone will get the hump and ask questions. how much you declare is up to your conscience.