A mark is the fall guy, dupe, target, or victim in, or of, a scam. In other words another carcass on the road to riches for the scammers, whether they be pseudoscientific or just plain criminals. In the TV show, The Real Hustle, presenter Paul Wilson uses the term in his pseudonym "Rob Marks" in reference to the fact he is robbing marks.
There is a saying in poker that goes "If you look around the table and you can't figure out who the mark is, it's probably you."
 Other (slightly more boring by RW standards) uses
- A common given name.
- The title/author's name of one of the four biblical Gospels.
- A sign or other distinguishing feature, whether real or abstract, such as The Mark of the Beast.
- The pre-euro German currency.
- A Pro Wrestling fan (Due to is nature of being presented as real when in reality it was pre-determined)