It depends on a lots of factors.

Would a gaming company be willing to give up a source of revenue from people buying ninja frags? While many aren't willing to think of a trading system as it's not part of their strategy, some can but they do it differently. The ninja refinement system in Naruto Online is the alternative that many games have but Tencent did not update this system so the interest to refinement is not there.

If an exact trading between frags, you really need to determine the ratio of trading. They will never allow 1 meta ninja frags to be equal to 1 elite instance ninja frag and they shouldn't cause there needs to be balance. If a ratio is established, do you really think Oasis would be capable of adjusting the ratios when meta ninjas stop becoming relevant? Look at some ninjas that still cost the same when they were relevant.

Sorry to say this but is even a worse idea since don't forget - people can make alts, so abuse would come. People would make secondary accounts just so they don't have to spend on the main account. If for some reason you are going to counter that no one would make an do it just for this purpose, you aren't an active player then. An active player would see the worth of creating an account, especially if the player is f2p. Usually its a trading system that trades into a currency before a trading system between fragments then the last choice would be trading between players if a company decides to add a trading system.

Also, you are also neglecting the fact the the gaming company would lose out on the 1 potential person who would spent between the two people. Both want the meta ninja and would spend to get the ninja whether is in in-game or real currency. Trading cuts the revenue about 50% if anything.

Trading between players is awful idea, it gets suggested every few months, and it will never work unless they put some big ingot tax on it so f2p can't use it at all, otherwise everyone will just use alts, if people bother to make alt for plundering they will definitly make some for trading.

As for tradings fragments for other ninjas, that already exists, you can trade fragments for Iruka or Chakra Naruto. Actually wouldn't be to bad if they added more redeem options there for different ninjas, but oasis won't do it, people crying to make sage based on power from first month of game and they can't even do that yet