I have some smaller paperbacks with what seems like white lines on the spine... Here's a picture of what I mean.
http://tinypic.com/r/15ml280/8

Is this kind of spine damage postable? They're all older books, but it might be questionable. Any feedback would be great!

Spine creases. Not a problem unless the spine is cracked open and pages are loose.

Most older books have these creases, a majority of the books (even newer ones) I received here have them.

Gail
 

Books with spine creases or lines like those shown in the OP's  picture are ok to post if the glue is not cracked and the pages aren't loose. If you can open a paperback to a given page and lay it flat on a table without holding it open then, even if no individual pages are loose, the glue and binding are cracked. Books with cracked binding may be considered as not being in good condition and result in RWAPs.

Haha. When I read the part about holding the book open and it staying, I panicked. I was like, "I've never had a book do that!!! Oh no!!" ...Then I finished the sentence. It's obviously bedtime, lol.

Yes, as already mentioned, your books are postable as long as the spine is intact and no loose pages.

Do be aware, however, that there are members here who dislike that on any books so they will have an RC against them.  Though that doesn't bother me, folks are allowed to set their conditions to be pickier than the conditions that make a book postable.