I have some bitcoins that I've kept for a couple of years and I'd like to sell them for a profit, without paying a big tax on it, and then to transfer the money through bank transfer to my country. Here in my country I'd have to pay 20%, which is not acceptable.
Well you still won't get any kind of exemption even if you sold your cryptocurrencies to a tax exempted country, taxation simply doesn't work like that since the tax that will be applied to you is the one where you are currently residing in regardless whether you have sold your cryptocurrency internationally. The profit/capital gains you will have from that sale of crypto would still be taxed at 20%, actually you might just be risking yourself to a scam looking for a buyer online on a international country with you having little to no experience with dealing it. My advice is for you to just pay your tax as I don't see any way around it but try to look if you have some kind of tax exemption or lower rates for assets held over the year since several countries have lower rates on long-term held assets.