If you have a stack of 10 items (number not important) and add them to the sell window but enter 5 as the quantity to sell, when you add a starting bid and buyout price the item quantity resets back to 10.
As I say, not a biggy but annoying as you can end up selling more than expected at a lower price per unit.
Noticed this on my Android devices (Nexus 7 and Galaxy S2)
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