i like this idea im not a dueler, but i like to think about how it will work.
so soras suggestion could be boiled down to two commands
1. the send command, to transfer currency, i don't see why it shouldn't transfer items too tbh
/send <amount|item> <player>
pretty straight forward, abusing this command to scam noobs with should not be an issue as you can maybe make it a toggle that is disabled by default and you have to go on the forums to enable it whenever you want, you can also put a massive disclaimer when someone enables it, you can do the same thing with the duel command too, im not too keen on "forum walling" features but if safety is a concern i don't see why not.
or maybe make it an in game inventory feature, so its clear that you are sending your money/items to someone else, and not just typing a silly command in chat.
2. the improved duel command, to duel currency or items.
/duel <p1 amount|item> <p2 amount|item> <wins>
this is an all in one command for all of your dueling needs.
if you want the classical dueling experience you had use it as follows:
/duel 20KTC
that will put everyone into spectators and they have to enter if they accept the duel.
if you want a first to 3 duel:
/duel 20KTC 20KTC 3
the first parameter being the amount of TC the player who entered the duel command is offering the second parameter is the amount player who enters into the duel is offering, this introduces uneven duels like 20KTC vs 10KTC not sure if duelers do that, could be fixed by typing it differently maybe.
like this:
/duel 20KTC 3
if you want an item duel first to 3:
/duel "Void Force" "Pure Relax" 3
item values vary alot, maybe tier lock it or just let players go crazy.
if you want item vs currency first to 3.
/duel 100KTC "Pure Relax" 3
or
/duel "Demon Force" 100KTC 3
could be TC fixed to shop prices,
if you want multiple items in your duel you just add a plus sign.
/duel "Demon Force"+"Void Relax" 200KTC 3
or just put them in a set and do.
/duel "Set:Duel" 200KTC 3
if you want to mix items and currency in your duel you just add a plus sign.
/duel "Demon Force"+20KTC 100KTC 3
i think this covers most cases except when players give someone a win advantage, maybe could be added like this.
/duel <p1 amount|item> <p2 amount|item> <wins> <p1 advantage> <p2 advantage>
its getting really silly now, sorry i had to go full nerd with this.