Very obscure job posting for faculty position. Pass onward, or keep to myself?
You are not obligated (unless by your own moral principles) to facilitate the distribution of job ads. 

You could take a broad self-interested perspective. In this case, you decide whether and how much you want the job and get a sense of your chance of getting the job without further distributing the job ad. Then, you think about how much you value others you care about getting the job and how much you value the profession as a whole making a better selection decision, and how much more likely that is to occur if you were to distribute the ad either to colleagues or to broader mailing lists, and also a sense of the cost in terms of your chances of not getting the job. From there, you can see what most aligns with your values and goals.

In general, I would think of job applications as a domain of life that is often governed by the principles of fair competition. By fair competition, I mean the idea that it's okay to focus on yourself. You shouldn't go out of your way to hurt other applicants, but that equally there is no expectation that you will help other applicants also (especially when you are competing for a single prize). Thus, if you learn information that might help your application (e.g., interests of the employer, what they're looking for, etc.), you are under no obligation to share this with other applicants. If you know about the job, and others don't, you are under no obligation to share it. The assumption here is that it is a domain of life where the ethics of competition operate. The assumption is that other "competitors" in some sense are also operating by these rules.

Anyway, these are two perspectives. My sense is that if you're very keen on getting the job and you think that you have a reasonable shot at getting it, that it would be reasonable to keep it to yourself.