With BfA winding down and most end-game markets trending downwards now is the time to craft cosmetic items. With no reason to spend gold on end-game items players will usually spend some gold on filling out cosmetic collections.
Use the lull to get recipes and stock up
Now is the time to unlock recipes for mounts and transmog and stock up so you can keep posting them the coming months and into Shadowlands. Once SL rolls around you will want to prioritize the new crafts and the new content, so using the time now when there may be little else to do is a great idea.
Crafted mounts should be priority one
My experience has always been that crafted mounts is the best type of crafted cosmetic item to focus on. They usually sell much better than transmog which makes them preferable.
Throughout the years Blizzard has added 17 tradeable crafted mounts to the game, with most of them coming from Engineering. Most of them sell quite well, and the list below is a great shortlist for recipes you can work towards.
Some of these are easier to get than others with the Vial of the Sounds being the hardest to get as the grind to get it is very RNG-heavy.
For posting mounts you can use my TSM settings for panthers, as it will work well for all kinds of crafted mounts.
Crafted transmog can also be a good source, although personally I would only focus on expensive items. There are a LOT of items in the game and if you try to craft every profitable 2k gold transmog piece you will end up spending an insane amount of gold on deposits and a lot of time.
What the right cutoff is depends on your gold, but somewhere in the 20-50k minimum price is where I would go. Some example crafts include the Felsteel Longblade, Phantom Blade, both of which have fairly rare recipes. Look through your professions to see if you have any recipes in this range, alternatively you can look through some transmog TSM groups to find the more expensive items. I don’t work much in this market personally, so I don’t have all the good items to hand sadly.
Recipes first, then restock
Right now I would work on getting some of these recipes. Craft one of each when you get the recipe of course, so you can start getting them on the AH, and then re-craft as you get sales. After that you want to get some more recipes. I would suggest doing a larger crafting session where you stock up 1-10 of each craft you end up getting that you can then slowly sell throughout shadowlands.
This should put you in a nice position to continuously profit.
Old world markets have complex supply chains
For a lot of these items you can save a lot of gold by doing intermediate crafting yourself. In particular you will want a transmutation specced alchemist and a elixir specced one if you want to do Vial of the Sands. If you just do panthers and engineering mounts you can get away with just the transmutation specialist, but if you have more characters at level 90 or above then get alchemy there as well for more daily cooldowns.
Good luck and have fun!