First thing first.
Choose a mastery and work with that mastery to make money.
Elixir Master will allow you to proc extra elixirs and flasks.
Potion Master will allow you to proc extra potions.
Transmutation Master will allow you to transmute extra gems, metals, elements.
Personally I have found potion master to be mostly useless unless you plan to make a lot of runic mana and runic healing potions. They are solid sellers and profitable. Elixir and Transmutation are the way to go.
Offering your services to craft for other people for a small fee +proc tip is a solid way to make money if you wish yo go that way.
Example for selling with proc: (works for each)
Elixir Master LFW. 1G per, +5G each time I get you a proc.
Elixir Master LFW. Free, you get everything you need, no charge and no tip. I keep procs.
Buying and selling.
1) Check the market for the prices of items used to make a potion, elixir/flask or transmute.
2) If the required items cost less then the assembled item you have the mastery for, buy them.
3) Make your items, hope for procs, list the finished product.
Potions to look for when reselling potions.
Runic Healing Potion
Runic Mana Potion
Elixirs to look for when reselling Elixirs.
All end game flasks
Elixir of water walking
Elixir of water breathing
Things to transmute.
Epic Gems. 1 per day, never miss a day. Buy the mats and make the gem. No need to check price, trust me, it will always be a nice profit.
NOTE: Do not ever sell your epic gem transmute. I see idiots selling them all the time and I buy them at any given chance. They charge 10g for a transmute. Worse case if I buy the mats for a total of 50 gold and pay them their 10g and sell the gem for at least 150g. Making myself a 90g profit for less then 1 minutes work. Do not be the idiot that sells your transmute for 10g when you could be making 90g off of it.
From my experience Transmutation Master is the best route to go for making money with the least work. At the bare minimum of work, doing the one epic per day, you will make at least 90g per day for 1 minute of your time to buy the mats, press the button and list it for sale. If you do get a lucky proc of 2 or even 5 each extra gem is pure profit.
Best case scenario is a 5 proc on the top cost gem. On my server that is around 200g. The mats for that top gem will be 70g meaning I buy the mats and list it for a 130g profit each day. If I get lucky with a 5 proc (only happens about once every other month. 2 procs are more likely for me at least) it becomes 800 additional cash made. 930g for 1 minute of your time. Even the most lazy of lazy people or the most casual of casual players can make money when it is that easy. Don't you think?