This is one of the most original ideas I've observed to reimplement dueling. I like the notion of a grand exchange-like system, and it causes RWTing to be very insecure. However, Real World Trading would still be possible, and there are several ways to circumvent the system.
The largest problem I see is the staking of items. A player buying a blue phat in an illegal RWTing sight might face some trouble getting their trader in a duel, but a player who asks a stake of a grim phat, adamant warhammer, and red bead might not. This combination would be so arbitrary and improbable that there could be little to no possibility of the participant not obtaining the RWTer he's buying from in a duel.
The best method to prevent this is to simply allow certain items to be staked, or not to allow stakes of multiple products. Money could be staked, and common items like whips, phats, ans h'ween masks (items traditionally staked), however the amount would be pre-set (players could not stake odd amounts like 4,397,031gp) and uncommonly staked items like colored beads couldn't be contained in precisely the exact same stake as big amounts of money or rare/expensive products.
Another problem I see with your thought is that several players would not enjoy the probability of it. As you would not be able to find out exactly what armor your participant was wearing, whether they were ranging or maging, or even their exact level (your thought outlines a group of near levels, but not an specific level) players may be afraid to stake large quantities, because the level of your opponent says very little about them.
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