I don't see why you can blame levelers. They added a feature in the game that increases the XP gained during a hunt, cool. But you have to PvP for it, that sucks. Well they just found a way to circumvent the system, which almost everyone does to a certain extent.
I would get mad at the Prestige System, not the traders. They are not breaking any rules, just playing in a less honorable manner than you expect.
Indeed, it's the system that is to blame. People will always "use" the system for their full benefit. It's the same in every area in the game, be it farming various medals, be it not taking undefended empowered relics, trading GvG and just about everything else.
I don't find anything bad with this. All levellers have basically two options - either you cooperate with your peers (with the plan of catching the ones above you) or you fight with your peers (with the plan of leaving them behind). Many people choose the cooperative way. Is trading PvP prestige any worse than sending surplus resources for brewing quests or offering advice/spoilers to somebody your level? People are using the system any way they find out, not just the way it was originally intended. Casting some buffs on your opponents to your advantage quite likely hasn't been intended - and yet it took a very long time before HCS modified this so that we now can block incoming buffs.
For the record, I have been asking Hoof for ages to solve this particular issue not by some silly limitations, but by removing the incentive for levellers to trade Prestige... The solution is very simple, rework the PvP prestige buff to provide some advantage that is of absolutely no use for levelling. This would solve the issue. (Well, maybe not, maybe it would only cause PvPers to trade PvP for prestige instead of levellers)