I don't see any legal issues with pitching your services to rights-holders. However, you may want to convert more than one book before characterizing yourself as offering a professional service.

You should also be able to either provide some type of OCR'ed text (as opposed to PDF) that can be converted to MOBI and EPUB, unless you want to learn how to do those conversions yourself and offer that service.
One problem you may have is in finding the correct holder of those rights. With these types of books, the copyrights were most likely held by publishers who are now out of business and cannot be located.
Also, you do have some competition, namely Google. They've been scanning books in various libraries for a few years now, and there's a good chance the books you're converting were also caught in the Google Books dragnet.
A third is that if you plan to charge anything for this, your product will need to be nearly perfect.
A fourth is that if you offer your services cheaply -- and yes, that is exactly what you're doing, since there are commercial conversion services out there for those who are sufficiently interested in going digital -- at least some of the people who take you up on your offer will make you absolutely and utterly miserable. In many cases, the people who are willing to pay the least also turn out to be extremely demanding, and a huge PITA to deal with. Just because someone is an author, publisher, or relative thereof, does not mean they are a good person or easy to deal with.
Books that are out of print, and whose rights-holders cannot be located by reasonable efforts, are known as "orphaned works." The copyrights still hold, though. Legislative resolution of the issues won't be easy either; the UK tried to pass a law that relaxed copyright restrictions on orphaned works, and many content creators (e.g. photographers) went through the roof and insisted this would make a huge loophole for them not to get paid. In the US, currently this is part of the furore around the Google Books settlement.
Keep some of this in mind while doing your next batch of conversions, and daydreaming about wider book availability....