What is the correct definition of "water marks" in the conditions page?

Please give me a duh moment,  I thought "water marks" in the conditions page meant if you had spilled something on a page or cover or the book got wet from rain or something and dried out, leaving a mark thus making the book unpostable.     However, my duh moment came when someone mentioned the brown spots that sometimes are on older books, not on the pages individually but when the book is closed you see random spots, usually brown or light tan  on the pages of the book when it is closed.    Sort of reminds me when I was a child and some of my friends would write their names on the pages of a closed book, seen when the book was closed but not so much if opened.

yellowing and aging of a book is allowed as long as it's not mold or mildew. 

Thank you Cindy,  I was wondering if what I described would be considered water damage due to a swap I made with a book that had the foxing on it.  The person was requesting a credit back because of it and I declined to return it giving my reason as to it not being water damage and the requestor did not have any personal conditions set before hand, however, the requestor maked the transaction as unresolved..I just hate that.   My books are for the most part  just read by me new unless I repost a book I received.   

"Foxing" and yellowing is allowed as mentioned above by others.  No stains of any kind (covers food, drink, ink, blood, etc.) are allowed.  Your requestor might mistakenly thought the foxing were stains.  Water damage leaves a books pages (usually) stiff or rough where the water was.  Also you can check when the book is closed and held lightly together whether there are any sections orpages that won't/don't line up with the others.  Sometimes dog ears or creases on a page will cause it to not be uniform in alignment, but the water/liquid would make it obvious and would most likely feel different to the touch (due to absorbing water).  Dog ears and creases don't make the pages feel different and you can tell from the fold lines what they are.

Ruth

There is a BIG difference between a watermark and a water stain.  I'm pasting the definition of a "watermark" from Wikipedia basicially it is a lightly visible mark to identify your book or to prevent counterfeiting.  It is a clean mark and is not considered damage.

A watermark is a recognizable image or pattern in paper that appears as various shades of lightness/darkness when viewed by transmitted light (or when viewed by reflected light, atop a dark background), caused by thickness or density variations in the paper.^[1] Watermarks have been used on postage stamps, currency, and other government documents to discourage counterfeiting. There are two main ways of producing watermarks in paper; the dandy roll process, and the more complex cylinder mould process. 

Were you actually inquiring about stains or water damage?

Watermarks in paper used for books is not to discourage or make counterfeiting more difficult. These do not have the same purpose as watermarks used on postage stamps or currency.

Watermarks on book papers, art papers and writing papers are used to identify the specific paper and paper manufacturer and are generally used only on higher quality papers.

Here is a description of watermarks in paper from www.watermarks.info for collectors

What is a watermark in paper?

Watermarks are designs or patterns put into paper during its production, by making thinner (line or wire watermarks) or thicker (shadow watermarks) the layer of pulp when it is still wet. Hence, the name.
    Paper watermarks can be seen holding the paper against the light or, in some cases, over a black surface. Usually, they show the manufacturers' name, and geometric designs, or images of animals, etc.
    The object of watermarks in paper is, essentially, identifying the paper, as a signature of the manufacturer, or as a security measure to avoid forgery of important documents as bank notes, passports, entry tickets, etc.
    Today, good quality writing paper, as well as art drawing paper or paper for bibliophile publishing, usually carry an identifying watermark. Nevertheless, it is easier to find watermarks in old papers from envelopes, letters, books... Indeed, watermarks are a good help in the study of old documents, as they can suggest their origin or date.

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The somewhat random spots, usually brown or light tan, on pages of (usually) older books or artwork on paper is referred to as "foxing".

Foxing is a term describing the age-related spots and browning seen on vintage paper documents such as books, documents, art prints, and so forth. The term is derived from the rusty red color of Reynard the fox and is a descriptive term for scattered spots commonly reddish-brown in color, but also applied to spots of other coloration ranging from yellow to black. It should be distinguished from visible surface colonies of mold growth, which may result in paper stains of a wide range of colors. Although unsightly and a negative factor in the value of the paper item for collectors, foxing does not affect the actual integrity of the paper.
 

Also watermarks are a publishing issue where liquid damage usually occurs post retail.

I agree with the others.  I sent out a book once which had some foxing on the inner cover (a collection of very small dots) and wondered if it was going to come back to bite me from the requestor (however, there was no issue).  I believe foxing comes from mineral deposits from the paper when it was made which slowly appear as the paper ages.  I used to work at a museum in the prints department and remember seeing it quite a bit.  If it was actually water damaged the pages of the paper would tend to become wavy after they dried.

Water marks are one of the defining factors for high quality stationery--- used by businesses also.