Actually, a lot of people make money in-game by selling Eden Crystals to others rather than using them. Right now, ECs are the most expensive they've ever been, you can get up to 90g each for them on Sapphire. And they're pretty much always in demand.
I know a few people who wait until there's a tiered spender, buy stuff like ECs or anything they particularly need from the Item Mall, then get the costume pieces from the tier and sell those. Though you usually need a high amount of AP to get anything good from the tier.
As an aside, I agree with Rockman - backpack space is a nightmare in this game xD