The Darkmoon Deck trinkets have been a supremely profitable staple of inscription for a while now. I farmed up the rank 3 recipe from the Darkmoon faire the last time around, and I have since leveled my Scribe to 110 and dived into the market. The Darkmoon Faire is going on currently and there is plenty of time to farm the recipes on your scribe. Even if it is not 110 yet, go get the rank 2 and 3 versions from the Faire so you can learn them when you hit level 110!
The Darkmoon deck market is awesome due to the relatively high barrier to entry. You need to farm the Darkmoon faire to get the rank 2 and 3 recipes, so few people will have them. You also need to craft a very large amount of cards to overcome the RNG so you get a decent amount of finished trinkets.
I have been stocking the decks for about 10 days. My TSM summary is shown below, and I have sales of 512 254 gold. This is about a 30 000 gold per deck average. My crafting cost for the decks with the rank 3 recipe is about 10 000 gold. So the profitability is amazing.
How does it work
The darkmoon cards and decks are completely unique. You craft the Darkmoon card of the Legion recipe, which gives you a random card from one of four decks of 8 cards. There are 32 possible cards. You need one of each of the 8 cards in a deck to turn them into the finished trinkets. The RNG component means you will have to craft a very large amount of cards to even get a single trinket.
I have sold 17 decks, which is equal to 136 cards and I have at least three times that amount in cards and finished decks. You need to craft enough that you can take advantage of the law of large numbers so you get a roughly even amount of decks. I suggest going for the finished decks as they well way faster than the individual cards. It is best if you can craft 100+ cards to get a decent chance of actually getting a couple of finished decks in your first batch. You can always supply with single cards from the Auction House, but that will cut into profits.
Farming the recipe
You obtain the rank 1 recipe from the level 110 Inscription quest Demon Ink. Completing it is completely straight forward. Professor Thaddeus Paleo in the Darkmoon Faire sell the rank 2 and rank 3 versions of the recipe. The recipes are sold for Darkmoon Prize Tickets.
The rank 3 recipe costs 250 Tickets and the rank 2 one 50, for a total of 300 Tickets. They are both bind on pick up so you need to farm the tickets on your scribe. Wowhead has a guide covering the Faire and how to farm tickets. On character can farm 281 tickets per character per faire. You can get 6 tickets daily from darkmoon game prizes that can be mailed from one character to another however, so making the 300 tickets needed in one faire is perfectly possible, even if you start at zero tickets.
You need a ton of pigment to keep a darkmoon card crafting operation going. As long as you have mass-milling it is usually better to mill your own herbs, and you can easily click mass mill and go afk. At rank 3 you need 30 Roseate pigment, 12 Sallow Pigment and 1 Felwort.
You get both forms of pigment from milling all Legion herbs, but the ratio varies. Milling felwort is the only way to get enough Sallow per Roseate to craft the cards. I have primarily milled aethril to get Roseate Pigment, but as far as I know Dreamleaf is also good. I just went by the TSM milling value, but it seems this does not include the nightmare pods. Felwort is the most important material due to being needed for Sallow Pigments and by itself.
I suggest making trade chat macros for buying felwort and whatever herb is best to mill for Roseate Pigment on your realm. Felwort is more important than the other herb as you need it straight up and it is not easily farmable. I run a crafting scan for sallow pigment daily to make sure I snag as much cheap felwort as possible. I can always turn any excess sallows into Tome of the Tranquil Mind or Legion Glyphs.
Selling the product
You have to main ways to sell your cards. You can sell the cards directly or wait until you have a full deck and sell the decks. Individual cards have wildly varying prices, as you can see in the screenshot below. Crafting price for one card is about 1200 gold on my realm. This can be profitable, but the sale rate is much lower than for the finished decks. The cards have an average daily sell rate of 0.08 compared to about 1.67 for the finished deck using the region average.
The advantage of selling single cards is of course that you do not need to craft hundreds of cards. I much prefer the finished decks, as the market is more obvious and I save inventory space on my bankalt. I have only tried selling finished decks so far for these reasons.
You can find my TSM settings here. The minimum price is set to 9 times the crafting cost of a single card. Normal and max prices rely on generic 150% and 300% dbmarket settings. I post one deck of each type at the time. The deposit is 1 silver regardless of auctioning time. I have it set to 24 hours, but 12 or 48 can be considered depending on how you feel your server is. My TSM group contains both the pre-7.2 item level 815 decks and the item level 835 versions. I expect the 835 versions to sell way better, hopefully I can unload my pre patch stock at OK prices.