I have a trade paperback book that's a bit wrinkled (I took it to the beach and it was jammed in my backpack). It's still completely readable and doesn't have any markings or anything but I don't think I can technically list it. Is there any way to put a note on it or anything to state the condition (I'm still new at this whole book swap thing)? 

There's no way to document anything about a book's condition.   If the pages are just wrinkled because of being in the backpack and not from water, then I would say it's postable.

Yes, as long as the pages are not ripped, wrinkled or bent is OK to post.

If you want to "neaten up" the wrinkled pages, try ironing them.

Or just stick it under a couple of heavy books (textbooks, coffee table) and leave them there for a few days or weeks, depending on how badly it's wrinkled.  Just be sure all the pages and covers are straight when you put it under there.

It depends on how wrinkled it is, I suppose.  There might be a point where you would no longer consider it to be in "good condition".  That would be your call.

I'd try everything possible under the sun to unwrinkle it and then decide if it is worth the risk to post it.