It seems wrong for someone to be able to buy XP from a dungeon, I say this as “Experiance” is supposed to be a reward for doing something where you can learn, you should never be able to buy it.
The first part of my suggestion is that the dungeon XP reward be given with the dungeon chest which should be able to be opened on any char on your account. This allows people to give the xp to the char they want (i.e. the human player has learnt from the dungeon, the gw2 char that gets the xp is not relevent as its controled by the same human). This is no different to now in that people can switch chars, this would just make it easier.
The second part of my suggestion is that the XP should be missing from the chest if the player does not contribute to the final boss kill – i.e. do reasonable damage. This should not be hard to implement as the framework to allow this already exists (some kills only give credit based on your damage in other areas of the game).
These changes would allow people who contribute to the dungeons to get xp as they should (without switching chars at the last min) but prevent people just buying xp which is clearly pay-to-win (as you can buy game gold with real money). It would also prevent people going afk during the final bossfight, which I have seen happen a few times.
I understand this may hurt people selling paths however I feel it is wrong to be able to buy XP, if you want to solo content thats great but nobody (players or Anet) should be able to sell levels – which is why level scrolls are account bound, i.e. you earn them you dont buy them.