Even if we accept nobody kept receipts or records in the early days of Bitcoin, aside from the blockchain, I can't think of a reason why someone running a legit business would find it "really impossible" to remember what they were selling.

Even if he couldn't really remember what his business was selling, he would still have receipts, records and/or emails demonstrating that the source of the 6000 Euros was a legitimate Bitcoin exchange, as a result of the massive increase in price.

Oh I know what I've been selling, but I can't really identify specific transactions. Nor do I have receipts, why would I? Most of the transactions are exploit sales over OTR/XMPP, possibly shady but not illegal.

This was also before the tax authority clarified on their stance on bitcoins, before that everyone was under the impression that you wouldn't have to tax your coins until you exchange them to real currency.

And that's ignoring all the coins I mined way back, it's literally impossible to attribute that transaction to any specific source.