All people claiming this is illegal are talking shit.
Iit depends on your local law. Selling bitcoin for euros is not an illegal practice in most areas.
Also, depending on where you do your banking, your bank will probably not ask any questions.
Earnings from trading and saving are not taxed in the Netherlands. We only pay 1,2% tax once per year over our total combined savings. So I can sell any amount of bitcoin for euros and no one cares at my bank. They happily accept my money actually. I'm not supposed to pay any tax over my trades.