How to cite marginalia in a second-hand book?
As far as I can see, you have 3 options: not using the material, using it without citing it and using it with citation. As you have no idea whether anyone is ever going to publish the ideas in those margins, not publishing seems overly cautious. Especially since you want to use the ideas there but seem to have no other source to credit.
Using without citing it seems like a very bad idea. Even disregarding the risk of the original author of those margins coming forward, passing someone else's ideas as your own is unethical.
That leaves citing the margins as is. You could explain the unusual circumstances in the publication you intent to use it in. If you have a personal webpage, you could publish photo's of the margins and also refer to those. Also you could simply offer anyone interested in those margins to contact you.