The exchange isn't even, it's a really bad exchange rate. If you stood in front of him you need 30 marks to buy enough materials for 25 reputation and only 5 marks back. I already showed the math earlier. To start with you have to begin with materials or marks. 60 marks or 100 materials to have enough to trade to get back enough for a second exchange. As I pointed out earlier you are also restricted by how many marks you can have and gain.

I know, which is why in my second paragraph, I said that even if the exchange rate was uneven, "buying XP" could still be done, even with less efficiency. IMO (and it is just that, my opinion), one should not be able to buy XP. And I say this as one that did it: just look at my Patrol stats. I even turned in Mats even when I DIDN'T get marks, because the XP was far more valuable to me. (Remember that XP doesn't just apply to your character, but also all of the gear you currently have equipped when you turn in/exchange.) I'm not saying it's a "one or the other" thing, I'm just saying that if they brought it back, the mechanic would probably still need adjustment. (EDIT: added note on XP and gear.)