Running is the ideal service to scam. That's how scammers will see it. Most other scams require some kind of stupidity. (i.e. not checking what the item is before you trade it).
Runner gets scammed is probably hardest. Because they can refuse to run you through the last area until you pay them. If however as a runner you're stupid enough to take them almost or through the last portal and they leave, it's a bit daft.
Runnees (or whatever they'd be called) are less likely to get scammed. As long as they don't pay until right at the end. Otherwise you pay, even if you're halfway through the journey, and the runner can just map-travel right outta there.
So unless you have a neutral person who can hold the money until the journey is completed on both parts, there is a huge potential for scamming right the way through it.
Of course, all scammers suck.
Best way not to be scammed? Play the game yourself. Don't wanna do the story with more than one char? Make 3 pvp chars. Or be very, very careful when choosing a runner, and pick someone you either know in the guild (so you can beat the heck outta their rep) or someone you know in real life (so you can go round their house and yell at them

)