Should we just ignore really yellow pages in books we receive? the one I got is really yellow and has an odor of "old books". Should I complain or just live with it?

Unless you have a specific requestor condition listing either of those conditions, I think you should just accept the book (without a problem). I guess if the pages were excessively yellow, you may have a case that the book isn't in good condition. But yellowing or odors by themselves (or together in this case) don't make the book unpostable.

There isn't anything anyone can do about the yellowing and the cheaper the printing the worse it seems to be. If you're getting older books you just have to deal. 

Yellowing of pages is specifically allowed by the rules.

I order a lot of classic sci fi and fantasy. I just got two today - one from '68 and one '70 - and they're both extremely yellowed and the inside covers and page edges are brown. That's just what happens to paperback books as they age. They're postable as long as there are no other issues.