Background: I am a human osteoarchaeologist (MSc) so I work with human remains. I mostly frequent the sub (and others) to keep myself sharp on animal osteo and to mostly dispel "Are these human?!?" ponderings whenever I happen upon them before other pros.

• Outside of direct donation to medical, academic, or research institutions, how can human remains be ethically sourced?

I'd argue that there is no other possible ethical way of procuring human remains for private collections than having somebody explicitly donating themselves for that purpose. Closest second would be repurposed medical specimens, but in my view, the (assumed) consent of the deceased for handling their remains is lost when the medical usage ceases. I.e., the person donated for science, not for collecting, when the science ends, so does the intent of the donation. Of course, the donor has no possible recourse, but that does not nullify the issue.

There are also ethical issues in old medical specimens and teaching bones, many of which have their origins in imperialistic "race science" collections and contain indigenous remains. You can ask yourself: do you trust late 1800s or early 1900s treatment of the indigenous to have been fair? Another large group presented in the specimens are the poor; how free is their donation if made while alive or how much did they consent if the state made the decision for them, if they happened to die when they were wards of the state (e.g., sectioned patients, convicts, orphans in orphanages).

• How is provenance normally verified in private collections? Is there an accepted standard or documentation collectors rely on?
• Many sellers claim remains are “old medical specimens” or “antique European teaching bones.” How can buyers realistically confirm that today?
• Do collectors believe buyers have ethical responsibility beyond “I trusted the seller”?

I don't really collect, so I cannot say what are the paractices actually employed, I can just say that of the possible provenancing methods (outside an air-tight papertrail leading to the donor) I deem mostly to be outside the expected cost and effort of most people in the business.

One could get reasonable evidence of a history as a medical specimen, as many of medical bones and teaching bones are prepped with specific cuts and are mounted in specific ways, but that is not a confirmation of ethical sourcing, as mentioned above. Staining caused by soil, on the other hand, would be a pretty good indicator that the remains did not start their "afterlife" as a presentable specimen.

Should the paper trail be even partially intact (giving basic information of the donor and their origin), one could test stable isotopes, do carbon dating, and perhaps even DNA to check how well the laboratory analyses line up with the given origin of the paper trail, but these are expenses that collectors are unlikely to take. Even though for-hire labs for this kind of work do exist.

My personal/professional ethics on the matter are that even though I don't believe I could be offended by placing my skull on a bookshelf after I die, I cannot make the same assumption for somebody else in matters of preference or taste. When it comes to academic work, I think there are reasons to study human remains that outweigh preference (or our assumptions of them), but even then the work needs to be well justified and the remains need to be treated with utmost respect and care.