I would also like to point out that the lower level items might be there as an incentive for lower level players to gather/craft items as well. Everyone needs a way to make some kind of money in game and if people don't want to gather/craft when they could just buy then a lower level person might be able to get said cash flow.

As for the FC house you posted, yes it's 36 slots... but that is technically for a group to do since you need four people to advance things along. Sure you CAN do everything yourself and that is fine but that is something you take upon yourself and really that is not the inventory systems fault.

As for the trading part, isn't it the same reason we can't trade more than one million gil at a time?