In order for bitcoin to work freely and legally in different countries, it is necessary to create an environment for buying and selling goods and services. When the tools are in place, then government agencies will be able to draw conclusions on how they can collect taxes from this activity. Such platforms should be easy to use for consumers and transparent for government agencies.
If the transaction happens through a centralized system then it is possible to tax based on the transaction. When transaction is made through a non custodial wallet to another non custodial wallet, it is quite hard to keep track of the person involved with the transaction. So, until the person himself files cryptocurrency holdings on his tax it is quite hard to find a solution on taxation.