I've seen this on my server too. There was one guy selling those Princess runs for 25G a pop, and that 25G covers however many runs it takes to get you the ring. So if it takes 5-10 runs to get you the ring, you still only pay that 25G one time. Really not a bad deal at all, considering that ring is a pre-raid BiS for a lot of people.
As a warrior main (not 60 yet) who eventually wants to go fury/dual wield and will be all about that ring, I'm going to admit that I've been tempted. Especially since that 25G also gets me the Princess kill/quest completion and a Thrash Blade.
And yeah, we're definitely going to see lowbie dungeon runs getting sold at the game moves along. You didn't see it at first because lowbies didn't have money to pay for stuff like Deadmines carries, but with more players looking to level/gear their alts now (and level 60 mains with gold to fund those alts), I think it'll start becoming more common.