Although it's very unlikely that KLab will ever implement trading in LLSIF, I've noticed that a lot of people like contemplating the idea anyway, myself included. I've tried to think of ways trading could be done without being abused. And who knows... maybe a KLab developer that speaks English will just chance upon this thread and like the ideas? :D Anyway, here is a trading system I came up with that seems pretty practical:

• Trading a card costs as much as idolising that card. That means it costs seals and gold.

(This should prevent most of the rerolling abuse, since both a rerolled account and a main account would have to sacrifice something in order to trade. The rerolled account will never really lose value, save for maybe getting a less popular card in exchange for a more popular card, and would have to be around long enough to get enough gold and seals. Also, this means that trading is only slightly cheaper than using the seal shop.)

• Only the same rarity card can be traded to the same rarity card. That means you cannot trade SRs for URs, and you cannot trade an unidolised SR for an idolised SR.

• Promo cards cannot be traded.^[1]

• R and N cards cannot be traded.

(This is to prevent veteran accounts from helping new accounts getting all of the Ns and Rs in the game right off the bat. This rule might not be necessary, though, since it doesn't affect that many people and doesn't seem a big deal to me.)

• Cards with different amount of skill slots either cannot be traded at all; or will, after trading, take the minimum number of skill slots among the cards traded.

(This should prevent people from making accounts specifically to spend seals so that their main account doesn't have to.)

The only thing my idea doesn't address is that event SRs can be traded for regular SRs. That means you could reroll accounts for SRs you want, and then T1 on your main for the extra third copy, and trade those for SRs. Since there's nothing in the code that distinguishes them, that either has to be added in, or maybe it isn't a big enough deal to be a problem (since tiering isn't that easy).

[1] I'm fairly sure there's something in the code that distinguishes promos from non-promos, since they're treated differently for skills.