1. open up the shop menu (using b) and select any item in the shop that you have in your inventory
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2. click sell amount and enter any amount higher than the amount the shop can buy (ie: 50000-stock+1 or higher), it doesn't matter if you don't have enough of the item
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3. click ok and it append the amount it could buy, even if we had less than the amount.
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it should replace the amount with the amount it could buy only if we had more than the amount otherwise it should replace it with the amount we have.