Currently, a gift box is 9 slots, however, in the markets, people are selling complete sets of clothing which can take up to 14 slots depending on which items are used and if accessories are included.
I have a few ideas that would help with this. a 4x4 box would allow us to did the outfits we want to sell in a new character's inventory without the need for a backpack, and still be able to fit 3 outfits in a cupboard.
alternatively, allowing to put a hand from one of the stalls into a slot in the wardrobes would offer more benefits as you could sell a COMPLETE outfit.
Using the box would allow the purchaser to know the stats on the suit if the seller desired to write it out, but with the wardrobe, the tooltip could potentially include the stats of the outfit and not have to worry about the seller mislabeling the box.