So, OP, you're question is basically how to avoid tax and not get in trouble.
You're from Greece, which means the EU, you're also not selling till $50 000 and if we start from the presumption it will be quite a bit of money I would advise you to not try even a p2p deal in cahs in Greece and this is the same for everyone else, not try it in your home nation and not in your hometown.
Get a cheap flight, arrange for a p2p deal in Croatia, Romania, Bulgaria, Hungary, or whatever as close as possible because of costs, and meet there with multiple! again, multiple traders split the sums. Or you can try finding some ATMs that still have limits for like 500E till they require KYC and visit 10 of them a day, sending each transaction from a different address so they don't red flag it.
And, be careful what you do with the cash after!
It doesn't matter how you accumulated it, the tax will be based on what you choose FIFO or weighted average and what proof you have you have bought that amount at x price.
And no, it's still capital tax gains, it will be 15%.
So if you have proof you've bought coins at $1000 and you sell at $50 000 you will pay only for how much you sell 15% of the 49k difference.
The 45% is the tax plus health tax plus pension fund plus whatever else is there, right? Not just flat income (wage) tax?
You're from Greece, which means the EU, you're also not selling till $50 000 and if we start from the presumption it will be quite a bit of money I would advise you to not try even a p2p deal in cahs in Greece and this is the same for everyone else, not try it in your home nation and not in your hometown.
Get a cheap flight, arrange for a p2p deal in Croatia, Romania, Bulgaria, Hungary, or whatever as close as possible because of costs, and meet there with multiple! again, multiple traders split the sums. Or you can try finding some ATMs that still have limits for like 500E till they require KYC and visit 10 of them a day, sending each transaction from a different address so they don't red flag it.
And, be careful what you do with the cash after!
You're actually correct; it's 15% tax on declared profits. However, I haven't bought Bitcoin myself; I've started accumulating from scratch; wouldn't the whole amount get taxed since it's basically a source of income?
It doesn't matter how you accumulated it, the tax will be based on what you choose FIFO or weighted average and what proof you have you have bought that amount at x price.
And no, it's still capital tax gains, it will be 15%.
So if you have proof you've bought coins at $1000 and you sell at $50 000 you will pay only for how much you sell 15% of the 49k difference.
In Romania I can just say it was earned through trading and will be taxed 10% capital gains tax and depending on the amount another 200-600 EURO on top for some other taxes. Not the same as income tax. Income tax is about 45% in Romania so it better not be 
))
))The 45% is the tax plus health tax plus pension fund plus whatever else is there, right? Not just flat income (wage) tax?