I can't seem to find the answer to this in the Help documents.

Are books with remainder marks (a dot or line on the top or bottom edge of the pages of the book) considered postable?

Yes.

Yes, absolutely. It is explicitly in the rules as OK. If you search help for "Book Condition Guidelines", you will see the full doc on book conditions that are OK and not OK.

From the Help Center:

Remainder marks are fine, just make sure that the requester doesn't have a RC about them. I had someone request a book from me and they had a RC about the book having no remainder's mark.

That's what I thought, but I was trying to double check before I posted some books and couldn't for the life of me find it in the help docs. I suppose because it's buried beneath all the textbook/cookbook exceptions.

Thanks everyone!

Edit: spelling.

I ordered a book from PBS back in September, and it came to me with a remainder mark.  I figure if PBS can send me a brand new book with a remainder mark, then it will be okay for me to post back into the system.

Is that why some books have a black mark on the page edges?  I never knew that there was a name for that! 

I hope anyone with a Remainder Mark  RC spells out exactly what that is, I had no idea.

Here's a defination of a remainder mark. Usually the books that are in the discount area's of  like Border's and B&N are remainder books. I've gotten several of them.

Remaindered books are books that are no longer selling well and whose remaining unsold copies are being liquidated by the publisher at greatly reduced prices. While the publisher takes a loss on the sales of these books, they're able to make some money off the sale and clear out space in the warehouses.^[1]

Copies of remaindered books are marked by the publisher, distributor or bookseller, to prevent them from being returned. "Remainder marks" have varied over the years, but today most remainders are marked with a stroke with a felt-tipped marker across the top or bottom of the book's pages, near the spine.

Only hardcovers and trade paperbacks (paperback books, often larger than "pocket" paperbacks, sold "to the trade" or directly to sales outlets) are typically remaindered. Mass market paperbacks ("pocket" paperback books sold through a third-party distributor) usually become stripped books rather than remaindered books. A book that might retail for $20 will typically be purchased by someone specializing in remainders for $1 and resold for approximately $5.^[