So basically Under World Trading, Investigating, and diplomacy; I heard slicing was good about a year ago.
Slicing is still decent. But for levelling, it's mainly income supplement from boxes in the world. Purple level 50 augment parts will be popular till the next tier rolls in.
And no, I would not advise Diplomacy and Investigation for lower-levels. They have some high-yield parts, but they're mostly endgame (radioactive paste, etc).
I would take a look at Treasure Hunting - it gives gift fragments, even at lower levels. Those don't sell as well as terenthium, but there's a market for them too. Even if you don't sell them, they're foolproof affection bonuses for companions. And affection means faster crafting/missions with greater crit chance (+5% crit at top affection I think).