BSide,
Good story, but a real bummer. Sounds like a nice book - can you post a scan or give us a link? I take it you bought the book from a "reputable" dealer?
The seep-through marker and "filled in" crease will definitelly earn you the "coveted" lavender label. It's way too young a book to have received such treatment - unless you were really taken on it.
I'd keep it and enjoy it, or sell it with full disclosure to a willing buyer. Having it graded may only be a waste of time and money, in that it may only end up reducing its value.
You might consider grading it and keeping it, however, as the tide may change someday.
I know that I'd still be interested in it.
cheers,