Mesos
NPCs:
Make sure you have a Miu Miu the Travelling Merchant on you (does not expire) as this is the easiest way to make money through selling equips
Selling useless equips dropped by mobs is a great way to get started however weapons are intentionally lowered.
A pet is a great way of saving time and getting money when you're training
Buying Pots can be expensive so make sure you're getting the most of your money. Try to get to that 1:1 ratio of Mesos to HP or even higher.
Limited Inventory Space:
In order to increase your slots, you need to purchase (6.4k NX) for more slots, 8 slots at a time or you need to level more in order to unlock slots
Thus with limited space, what do I really need to hold onto?
Specific ores (Diamond, Black Crystal, Gold and DEX/LUK/Power Crystals)
Popular 10% scrolls: weapon attack scrolls, overall scrolls, chaos scrolls, clean slate scrolls, etc.
Attack potions (Ciders, Primal Brew, Naricain Elixir, Onyx Apples)
Highly-sought after Mastery Books (Genesis 20/30, Triple Throw 20)
Trophies and GMLs
Even More Mesos:
Mia in Lost Memories sells FM merchants for 1 trophy for 10 days or purchased though the cash shop
Well scrolled/levelled gear is always wanted on the market (check discord's market) such as Facestompers, Brown Work Glove/Stormcaster Glove, Blackfist cloak, Face Mask, Angelic Blessing aring/Dark Angelic Blessing Ring, Silver Earrings, etc.
Certain skill books are always in demand