If you want to be legit you'll take care of these aspects. For example, I bought bitcoin by moving money through a bank wire to a well regarded bitcoin exchange (Bitstamp). The same for cashing out of bitcoin. So I have a clear money trace.

If you mined or gained your bitcoin by buying it through local bitcoin, the taxman might give you some trouble, but generally, the presumption of good will and non-guilty still applies, meaning that if they don't agree, it kind of their job to prove that you are guilty of something (ie: you got your bitcoin through ransomware)