"But that thing should be easily prevented by introduction of some rules"

Not really, even with all that rule, people still could abused it. Some people could make a lvl 100 account easily, it just takes some times, but after they reached the lvl, they could abuse it eternally. And even if they didnt make a lvl 100 account, they could get some account from the player that quit the game, and another abuse will happen.

Example of abusing trading : some second accounts somehow could get a rare fragments from event, for example getting a rare ninja in lucky star event, and then they could sell the fragments for just a mere 1 coupons each fragments to their main account. Or make main account sell a normal fragments, let just say normal naruto fragment, for 1000 coupons (since according to your rule, its possible to sell 1 fragment for 1000 coupons), and make their alts to buy it.

Please to any mod that read the suggestions about trading, please never accept it, it would ruin the game eternally. There is no rule that could prevent it.