I bought this book used and the edges are yellowed.  I don't know if it's a stain from liquid, or sunlight or what.  Is it postable?

I trade a lot of old paperbacks. It's common for the pages to discolor over time. 

If it's a liquid stain, there should be other damage along with the staining, like warped pages, uneven yellowing, water marks, etc.

If it's just normal age-yellowing, it's postable. Water damage would be a no.

Water damage usually changes the texture.  Yellowing due to sun and the aging of the paper acids would not change the paper texture.

They also used to dye the edges of paperbacks in yellow, or blue, or green. It could be that and if so, it is not a concern.

Thanks to all of you.  That helps a lot.