Laws are not always crystal clear in each case because to do so risks making them capable of being worked around (and of course in some cases they are just badly drafted - but I don't see this so much with GDPR). Laws are then subject to interpretation by the courts and by lawyers. If you're having issues with understanding laws, then you may need an expert to guide you, as in many areas of life.
Recital 23 of GDPR will give you insight into how your Zimbabwean guitar pick seller would be treated. If they are consciously offering picks to data subjects in the EU, either through specifically referencing EU data subjects, or through offering picks in EU currencies or tailoring the site for different European languages, then they are likely in scope.
Conflict of laws provisions are a separate point, however in various areas, the GDPR expressly states that legal obligations override GDPR obligations in various areas.
Whenever any company considers that a law may apply to them (whether as a result of operating in the country or because of the extra-territorial implications of certain laws, like GDPR) they generally take advice from local lawyers as to the implications or do independent research.
The regulation is obviously available and there is a host of interpretative guidelines issued by the Article 29 Working Party which will enable anyone with enough time and desire to understand the implications of compliance. I'm not sure what kind of assistance you're looking for here? It's incumbent on the party who wants to operate in a country/provide services to users in that country to understand the relevant laws.
If you disagree with the extra-territorial application of the GDPR then that's a separate issue. Bringing international tax treatment into the discussion is also not of relevance.
