The idea is nice. If you need quick TC, you can sell an item instantly for a low price. If you are worried about that it can be abused, the price might be based on curent market price, not the shop. So you will be unable to buy items from market to resell them for more just with this option.
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Even though that, it would be more safe if the system would remember the price that the item was bought for, and discount the quicksell price from it. So let's say an item costs 2k in market, and the next guy in market offers the same item for 10k. Some person wants to make quick profit - buy the item for 2k and quicksell it for 5k (if it's a 50% drop), using the quicksell option he still will sell it for 1k.
IMO this is a suggestion that should be developed and implemented.