http://en.wikipedia.org/wiki/Mark_(victim)
This is an underground slang term that evolved amongst swindlers because the intended victim was said to be "marked" for the sting. It could also be a reference to confidence man Ben Marks of Council Bluffs, Iowa whom David Maurer credited as the originator of the "Big Store". The victims were, in actuality, Marks' although the reference would have meant nothing to them.
In some instances the victim would literally be marked. This was a practice among traveling crooked carnival game operators in the past.
Coming from a somewhat carnival background in pro wrestling that's close to the term. The term Mark (at least from oral tradition and the old timers I've spoken to) would refer to the person with the money. Old time wrestling was fixed but to the specific town. For instance the wrestling crew would get an old collegiate wrestler who would travel to the town the carnival was going to be in sometimes up to 3 months ahead of the carnival and be sure to be seen at every event the town threw, get to know everyone and make sure they know he's a former collegiate wrestler. Then the carnival would come to town with the wrestler making an open challenge, they would have a journeyman take a dive, and possibly a more skilled journeyman take another dive. Here's where the term mark comes in, they would have a plain clothed carney behind the folks making bets. The one with the biggest wad of cash the plain clothed carney would go behind and make a guesture pointing him out to the wrestler. "Marking" him to say, he's the one they're going to be taking the money from. Whether a simple pick pocket job, or the more elaborate plan. This is the fun part, the collegiate wrestler happens into the tent and gets in a verbal spat with the wrestler, now the town knows he's a former collegiate wrestler and eggs him onto challenging him. They play the crowd into being convinced one will win, the plain clothed carney talks the mark into wagering everything on the collegiate wrestler and wham, the collegiate wrestler takes a dive and everyone walks away the con with a big fat wad of cash.
Now it's a little different, we'll mark someone who is easy to work in the crowd to rile up the rest of the crowd, or we'll mark someone with kids who are complaining about food and stuff to get them to buy more food, or my personal favorite marking folks who buy alot of souveniers and hooking them into buying all my merchandise. Ain't the con-game grand?