There are two sizes for Buy Box, half block size and full block size, they are both unlock at level 4 in the shop. Half block size cost 6 plat in the shop with capacity of 250 items, full block size cost 8 plat with the same capacity, limit by player inventory size.
Place items in the box you want to sell. Another user can later purchase the entire contents of the box for a gold fee that you have set. You can later retrieve the fee from the box.