It's a vicious cycle, but the starting point are the prices. You don't "start" by buying gils then purposely pay ridiculous prices just because you can.
The question is why were the prices set so high in the first place ? Before housing, there was literally nothing worth buying.
If you want to keep the economy in check, you need to have a limited amount of gils and supplies. Crafting guilds should not be able to supply infinite amount of materials, vendors shouldn't sell infinite amount of items and have infinite amount of gils to buy from you.
But putting such a system is very difficult so, the economy is sure to derail at some point...