If you're looking for a way to make money while you level, the best way to make gil is probably stacking and selling crystals.

As easy as it is to level up in XI these days, it would be almost impossible to keep gear and spells current solely by selling things you find along the way though.

If your account is level 20 and 2 weeks old or more my suggestion is to take up fishing. Make do with the gear and spells you have or can get easily until you hit level 20. Head over to Southern San d'Oria and get a few stacks of little worms from Lusiane at F-8. Then you can head out to West Ronfaure and kill the Goblin Fishers that roam around the Knightwell pond to the south until you get a Yew Fishing Rod. Equip the rod and bait and start fishing.

You're capped on how many fish you can catch a day and it'll take a little time to get used to the stamina bar, but after a little while you'll know when you have moat carp on the line. Last I checked they sell for around 10k a stack and if your account can catch the limit then you can make 150k in a matter of hours. That should easily be enough to cover your spells for a while...