Just put items into a "coffer/bank" that players are willing to lose in BH.

(The items are examples, it can be any items/cash stack)

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Player A has: Whip, Sara Sword, and D bow in the coffer.

Player B has: a barrows set, uncharged trident, and ranger hat.

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If player A kills player B - He now has an option to choose ONE of the three items in the bank/coffer of player B.

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There will never be a method that isn't exploited by bots unless the actual playerbase has skin the game for the reward system. This is an upfront cost to only the players. It would make 0 sense for bots to do this since it will require start up cash ON TOP OF being able to match the value of the opposing player. If bot farms were to do this - they will never gain value, only recycle value and stay constant.