From Weapon Merchant Aragas you can buy multiple of the same weapon in one transaction. This is rarely needed for niche purposes such as buying multiple duplicate weapons to increase their enhancement cap to 35 or multiple of a weapon type as enhancement or augment affixing fodder.
This also confirms that you can buy multiples of non-stacking items (weapons don't stack) in one transaction.
Similarly the Pet Supplies Shop run by Pet Lab Technician Ariel you can select multiple items to trade to get multiple of every option from Crisis Roll down. You may need multiple of some of these on rare occasion as well, most obviously for Caramel Crusher to crush 8 caramel cubes in a sweets box.
However the two trades at the top of the list for EXP Egg 6★ and EXP Egg 12★ never lets you select more than 1 of the required item (Grinder and Photon Sphere respectively). This is in spite of the fact that you generally want to buy these eggs in batches of 5 at a minimum for every pet you're raising.
As such you need to press the confirm button/key four times and then tap right to set 0 to 1 for every single egg you buy. When you're trying to raise 4-6 pets concurrently, this requires repeating the transaction 20-25 times which is obscenely excessive and completely unnecessary as we could just be able to select more than 1 source item as we can with every other item from the same shop.
And this can't be due to EXP Eggs not being able to stack, because as mentioned above you can buy multiple weapons fine.