This book came from a thrift store, in good shape, except a black magic marker'ed X on the front cover. (That is how they seperated their normal books from the ones they sold for a quarter.) The X does take up the middle of the front cover, but it has faded over the years. It doesn't obscure any text, and you can see the cover art easily through the mark. It is an older book (1966) not otherwise listed on Paperbackswap. Can i post this book in good conscience?

Yes, you can. I've received books with marks on the covers (often deliberate, like a heart or star drawn on), and at first I thought it would be RWAP but never saw anything that said no writing on covers in the Help Docs, just the insides can't be written on.

In my opinion, the book is postable.  Your X is a "remainder" mark, or how your bookstore indicates that the book is one of their discounted stock.  Other remainder markings (on edges of book pages), cut corners, library decirculation stamps, etc. are allowed, so this is allowable as well.  The prohibition against writing for books specifies writing on text pages, not covers.

That being said, I would probably PM the requestor in advance to let them know about the marking, and give them a deadline by which you need to hear back from them or you are sending the book as requested.  (If they don't want it, even though it is postable, I would advise in PM that they should be the one to cancel the transaction, so the book isn't removed from the system and since it is their decision to reject.)  If the transaction isn't canceled by the transaction deadline, mail the book.

(Other members may advise against sending the advance PM.  Many members feel that no communication between members should be necessary to transact a swap.  I'm one of the other persuasion, that although not necessary, better communication between members fosters better feeling about swaps...)

 I believe it to be postable.

PMing about it before mailing is up to you but if you do PM give them a set time like 2 days to respond telling you not to mail else it is going in the mail box just so things are not left hanging.

You could try to get the marker off, if you want. I bought some books at my Borders closing sale and was able to wipe off the permanent marker using a cotton ball and some hair spray. I've also heard that rubbing alcohol will remove permanent marker as well.

I agree with everyone else here; there can be no writing in the book on text pages (exception: textbook rule) but nowhere in the rules does it say anything about writing on the cover. 

In my opinion, if it's a paperback then it isn't postable. Posted books shouldn't have any stains or writing of any kind. If it's on the dustjacket, it's fine since dustjackets aren't required. If it's a glossy cover, a cotton swab with rubbing alcohol will remove it without a trace. But I guess it all depends on the receiver's opinion. Anything iffy shouldn't be posted though.

I feel it's postable, but that it falls into a gray area -- the help docs say no writing on text pages, and specifically allow a signature or note on the inside cover, title page, and title page.  They don't mention anything about writing on the cover being okay.  And this isn't exactly writing -- it's a big X.  So I agree that it's a good idea to PM.

The Help documents specifically delineate the condition of cover pages from text pages.  There is no requirement that cover pages be free from writing.  Although the reference is to page edges, remainder marks are specifically allowed.

It seems minor to me and pesonally am impressed that you would take the time to ask.  I would send a PM to whoever requests it...  I requested a book and received a PM about a mark on the book and I really appreciated that effort.   The book came and was in great condition.  But you know, I've seen that some people on here are very easy going about these types of things and others are a bit more rigid.  If you don't say anything and send it to someone who is more rigid  you could find yourself in an unpleasant situation.   That being said, I don't think anyone on here should expect to get "brand new" books - this is, by definition, all about used books.