Most organizations I've been with (US) always have an IP clause so it's highly likely that they had that somewhere.

@JosephDoggie, and in that case the issue is a contractual one. Do some institutions have requirements that professors aren't allowed to write books and sell them or must turn over royalties to the institution? Not my experience over 40 years. Patentable inventions are a different matter, of course.

Seconding @Buffy's comment: in the US, in math at least, apart from patentable things or start-up companies, papers and books written by faculty are their own intellectual property.

Before committing to the "cuppa coffee" idea, it may be prudent to look into any applicable policies about receiving gifts of monetary value. Even if each gift is small enough, there may be an annual limit.

What about YouTube videos? Can one make the argument that the creation of YouTube videos is essentially writing books but in a different format and therefore acceptable?

@Timmy, perhaps, as long as no university resources were used in the production. It is a more difficult question unless you make those videos also available (free) to your students.

It's interesting this answer is a polar opposite to your opinion on professors asking for tips in another question, where it's described as a "moral failing" (academia.stackexchange.com/questions/213926/…). I agree there's some differences in the scenarios, but it seems like a fine line as to when it's appropriate to seek additional compensation for things directly adjacent to your job description. How is something as basic as posting lecture notes with "buy me a coffee" different from asking for a tip?

@NuclearHoagie, actually they are completely consistent. The "tips" would come from students. The "cuppa" would come from people for whom you have no obligation. Your job description (and contract) is unlikely to require that you give your IP to the general public.

I don't know the situation in the USA, but in Germany, professors are civil servants and are therefore, I think, prohibited from soliciting or accepting donations.

@gerrit, that changes things of course. But I suspect that things produced by professors are not automatically "public domain". No copyright, nothing patentable... In the US, things produced within the government, and some federal labs, have that characteristic, but not professors.

But I think they're not prohibited from selling stuff or in getting compensated for expenses, which would lead to the question: does providing something for free and then asking for voluntary contributions count as selling or as soliciting donations? I can imagine courts might disagree over this question in Germany. It might depend on how the donations button is presented in practice. Now, Germany is very different from the USA so this does not help the OP any farther at all, unless similar rules apply for the professor in question.

Are you saying it's fine to ask for tips, so long as they don't come from students? The problem is that I'd expect the vast majority of the people who wind up on the page for the professor's lecture notes are, in fact, students taking their course - the professor is, by and large, asking their students to buy them a coffee. The lecture notes are also posted to the university's system, so this seems pretty clearly within the remit of the job. Some aspects of this (lecture notes in particular) seem quite close to asking students to tip them for performing their basic job duties.

@NuclearHoagie, please read everything again. The students have an alternate official source for all the materials. They don't need to tip and are unlikely to even see such a page. The "ask" is for other people, not students. It would be wrong if the materials were not provided alternately. I don't expect the vast majority to be students who have other, official, sources.

I agree with your overall argument, but I think it’s not true that “[students] are unlikely to even see such a page” — In my experience students are often just as likely to look for study resources by googling (leading potentially to the instructor’s public-facing page) as by going through the official course website.

@NuclearHoagie, the action in question appears unethical to you. Could you please elaborate on your perspective as an answer?

Even patents return some money to the professor/inventors. It's a 50/50 split with the institution where I've been.

@user71659, it probably varies a bit, but yes, sharing is typical. And the institution assumes the cost of filing and of defending any challenges. Those costs are steep in lawyer fees.

@Timmy I don't necessarily think it's unethical. I just find it interesting that in the "should professors be tipped" question, every answer unanimously agreed it would be a major conflict of interest for a professor to solicit payment from their students. But the unanimous opinion here is that soliciting donations from the general public is fine, even if some (or many) students see the link and do, in fact, pay their professor. I'm not sure if this just needs qualification - that getting paid for your knowledge is OK, but getting paid for it by your students is not.

@NuclearHoagie I think I can see your point. There are some similarities, but I still think the two situations are quite different.