I'll tell you what I do, granted as mentioned by Bulwarkz this works best on a better populated server, try and look at the items that are empty for crafts you are leveling that have nice stats and people are inflating the prices. You're new enough so ask yourself, "What would I pay for this?" and go from there. Some items will also sell better to NPCs rather than PCs. A lot of the 50 crafters in my LS ... they have MILLIONS in gil by simply doing just that; selling to NPCs. I personally try to sell a few things I know lower level players want/need and sell for decent profit without hurting a new player's chance to afford more gear.
Good Luck! :D
