Honestly, any of the DoL will do it if you watch the market.
The easiest would be Miner or Botanist, you can sell raw mats, crystals, shards, etc for a nice price.
Fishermen is a bit tricky since many things don't require the use of fish nearly as much as say, Iron Ore.

Bottom line, go with the one you like best. If you watch the market and are careful about how much you undercut you will make money.
I found that selling smaller stacks or singles will get you gil faster.
For example, I sell Gold Ore in stacks of x3 because you need x3 of them (plus x1 Copper Ore) to make a Rose Gold Nugget.
It sells a lot faster because if someone only needs one or two nuggets they don't have to buy 20 gold ores.
Likewise, if I sell gems, I will sell them in stacks of 5-10 and singles.

Another thing you can do is sell items that people use for leves. When I sell Ignus Snails, I will sell either singles or stacks of x3 because you would need 3 Ingus Snails to do a turn in leve.

People will pay for convenience.
Another thing you want to do is make sure to check the sale history on each item. That way you will know the average price of an item and you can price your accordingly.

Lastly, if you level your retainer as a botanist or miner, you can send it out to collect crystals and shards via ventures. ^^
Best of luck!