Which market has made you the most gold over the years?
The honest answer is I have no idea. I tend to sell whatever I have lying around in my bags or I've got a surplus of in the bank. As I really don't have a clue what the honest answer is I'll talk about the top 2 markets I usually play as these are most likely to have made me the most gold.
Selling gathered profession materials
By 'gathered' I mean herb and ore/bars that I've collected and for ease will also include cloth. I've always sold any surplus of these materials and found that it can be a good way of making gold. However, the quantity and price at which you can sell these items really depends on the amount available on the market and demand. Some days its easy to sell a stack of 20 ore for over 200g, aother day the same ore will make maybe 50g. It's safe to say this market is very unpredictable but one I've found to be worth playing.
Homemade pets & mounts
Theres a hole range of in-game pets and mounts that can be homemade and sold for a profit and I've played this market since I was able to make my first pet. I think this first pet was the Pet Bombling made by an engineer. I have made duplicates of every pet that I'm able to make and sold it. If the market price is low and I won't make a profit then I keep hold of the pet until it will. Concerning mounts I sold the first Mekgineer's Chopper that I made. I managed to make a tidy profit that paid for half of my own chopper.
Aside from selling gathered materials and pets/mounts, I try to sell whatever I can - from gems to food. Not everything sells but I think it's always worth a shot.