The fact is, they are all just different shades of gray.

I doubt there is anyone anywhere that can tell you what is 100% legal, and what is not. It depends on many factors, such as what you buy, how much, who it comes from, what jursidiction you live in, and how a given authority interprets our ever changing laws and regulations.

Buying from an OP with appropriate medical records and consults is certainly going to be "darker gray" than buying from places with only a questionnaire.

IMHO, common sense is the key. If it feels wrong, it probably is.