It is pretty easy to make gold on a high population server. There are so many players that anyone needs almost anything and anything can be sold to almost any price.
Even though you can make gold on a low population server, it is more difficult. You have to use another tactic.
First of all, you can forget the idea of making ten thousands of gold a day, like you can (or should be able to) on a high population server. You have to work in other ways.
I have several characters on both low and high population servers, and I have seen and experienced the differences. Instead of complaining (and moving), I tried to understand the way things move and work on a low population server and how I can make gold.
Now, you have to understand one crucial thing about low population server: They generally don’t attract new players. The players who are there, are players who have been here for years, and might have an army of alts. So you can forget the niche markets at all.
Second thing: There are fewer potential buyers (logical!) so, again, forget niche products. Forget transmog sets, etc. This will not have any interest.
If you are really lucky to get a very, very rare drop (like a BoP mount), you might end up selling it, but, here again, be prepared to list it, and relist it, and relist it… Not sure it is worth it.
So, be prepared that making gold on a low population realm will take a lot more of efforts than on a high population realm. There is just less gold in game so therefore, there will be less ending in your pocket. These are the rules.
Therefore, the most important on a low population realm, is to focus on selling items that everybody needs: gems, enchanting material and such.
But of course, you will need more than gems and enchanting to make gold. Especially that you will have to always undercut to be able to sell. On a bigger server, you don’t always need to undercut common things. People will, sooner or later, come to you. But on a little realm, you have to because there aren’t that many buyers.
Here are a bit of advice for each profession.
Alchemy: Since the Golden Lotus prices have gone down (because now there are more high levels able to farm them), flask are profitable, no matter if you are transmutation master or elixir master. Trillium bar sell very well. If the price of 10 ghost iron bars is lower than one trillium bar, then you will make a fortune (especially if you are transmutation master). Remember that you can get ghost iron and trillium from Samsoong Ranch.
Blacksmithing: Forget it. The only thing you will sell are belt buckles and on a low population server, it isn’t that needed. Yes, when they made PvP gear craftable in 5.2, it helped at first, because people wanted their gear-level up, and it might be possible to sell on a PvP-realm. However, if you are not on a PvP realm… forget it.
Enchanting: That is a must. You can make the daily Sha Crystal and you can make a shuffle: Make armor/weapon/rings/necklace -> disenchant -> sell. You will make your gold mostly from these, not much with the reputation-locked recipes.
Engineering: Worthless, as usual. It is good for yourself, but especially since they removed ammunitions with Cataclysm, there is not much gold making there. Yes, you can make a couple of mounts, but the few that can be used by a non-engineer are so expensive to make, that you will probably not find any buyer. So engineering for yourself, yes, it is fun and okay, but to make gold: NO!
Inscription: Glyph don’t sell. Not for much, so don’t expect the big money coming. Darkmoon cards might sell a bit. Some people might be desperate, so you might sell a few. The best, right now, is the shoulder enchants. It might be an okay profession to make some gold.
Jewelcrafting: This is a pretty okay profession, especially to make jewelry to disenchant. With reputation with the Order of the Cloud Serpent, you might make nice mounts that aren’t too expensive (if you aren’t too greedy), and they might sell.
Leatherworking: Difficult. You can only make and sell few enhancements (a bit more if you get honored with Golden Lotus), so you might make a bit of gold there.
Tailoring: It can be either great or a fiasco. It depends if you do the dailies with your tailor or not. You can learn the Royal Satchel (28 slots bag) once you are revered with the August Celestials and that might be worth it. Few bags might sell… or not. So you can either get a lot of gold from the different bags or if there are a few tailors with these recipes, it might not be worth it. You will have to examine your server’s AH to know more about that.