Money making in Lunia can be tough at the start, mainly because most things you can get are worth little to nothing. So can you do to make money and what should you sell? Here is a very basic and incomplete list of things to consider, keeping in mind Square 1 is currently the best place to set up a shop.

The Sell Command
If you do not have a shop, the best way to sell something in a square (barring the Shout of Authority :P) is to use the sell command by preceding your sell message with "/v".
Equipment
Most times, it's best to extract any equipment lower than 5D, since the time spent in that level range is not enough to justify spending any money on equipment, so most people will avoid making low level purchases. Determining the value can be very tricky, since people wil always have different opinions; pricing them is something that will come with experience.
Generally, if it is a rare item, has a positive str/int/vit grade, and/or upgraded, it is worth more than normal. The best possible grade you can get (IMO) is a damage grade. If it is a negative or neutral grade (Defense, DEX, etc), it is not worth it's normal price or less. Due to this, grading can significantly reduce or increase and item's value.

Fortification Stones and Catalysts
Since these remain relevant throughout the game, these are a decent source of income. Gained from extraction of equipment and Loose Boxes found in stages, they are usually sold by stacks of 250.

Shining Jewels
Dimly, Shining, Brightly, and Cold Shining Jewels are something of good value in Lunia Gold. All of the crafting jewels can be obtained through stages, quests, or crafting (shining powder). Only dimlies can be obtained through extraction.

Reagents
These also remain relevant throughout the game, so there will always be a demand. You can get a stack of 50 of any reagent from Parne (north of Square), or farm these from the Hidden Elf Garden side-stage.

Rare Jewels
Adamantium, Tantalunite, and Rutheniem: By trading 10 beads in to Duran you can choose any one of them. You may obtain one colored bead per day per fish type from Duran.