I have inherited several boxes of old books mostly Hop A Long Cassidy and The Saint from the 1920 to the 1950's.  Some are seriously not postable but other I am just not sure.  Several of the hardbacks from the 1940's have pages that have turned not yellow but more of a brown on all edges of the the pages.  The books are still readable and the dark coloring fades out before the words start.  Some of the hardbacks have a few strings from the fabric on the covers coming off.  What should I consider when posting books this old?  

Strings from the fabric of the covers is not a problem.

Yellowing is not a problem, but your description makes it sound severe. A photo of a typical page would help.

What Rick said.  Postability is the major concern of course.  In addition, this may be a situation where you wish to pm regarding condition, so there are no unpleasant surprises.  But as long as the spine is solid (and the books don't have oodles of more current reprints) requesting members tend to be more forgiving on condition of these vintage OOP books. 

What you perceive as brown might really just be extreme yellowing, or even foxing if the coloration is inconsistent.  It might also be possible that something about the environment in which they were kept might have an effect on the yellowing process; if they had once belonged to a smoker, for example, it might have contributed.  I've got a collection of older books, and plenty of them have darkened pages that go beyond the simple yellowing of a '70s era paperback.  You might consider posting the books that meet the guidelines and advertising them in the Book Bazaar, which would not only give you a chance to explain their aged condition, but might also get them to move more quickly off of your shelf; plenty of members (such as myself) keep an eye out for vintage books.