Yes! Imagine you bought 1 litre of gaseous milk. If you transferred it into a 0.5 litre vessel, the milk's new volume would be 0.5 litre.
So, it doesn't make sense to sell gaseous substances only by their volume. This is simply because they don't have a fixed volume! For example, if an oxygen tank just said that it had 20 litres of Oxygen, we wouldn't know if there was enough Oxygen in it to breathe for 5 minutes. There may be very little Oxygen, spread in a huge gas cylinder!