Like i've stated in many cases in the past, my favorite way to turn a profit, or make a killer sale is the one where i have to invest the minimum time into it and get the maximum profit.
One of the ways that i see are easiest to get to this is simply by using your professions to acquire raw ore/herbs then smelt/crush them and re list them back.
Main reason why this is a good idea is, well because everyone that's playing the game for the PvP or PvE aspect of the game are choosing their professions that give them the edge in competitiveness and maximum amount of DPS/HPS increase.
Well the good thing about this is that even with those professions they will still need someone with the gathering professions to actually turn their ore, leather or herbs into the materials they will need to have to continue working with their max professions.
As a good example for this would be the price disparity between the ore/bars in many of the mineral tiers. We can see on many servers that prices of a given ore is (as an example) 10g/stack, and some of the bars require 1 ore or 2 ores to smelt into a bar, but for some reason the price of the bar for that same ore that requires 1 ore to make it into a bar is 15g or 20g. Well what that means is that you can turn a profit by simply buying out the ore stock ( i'm talking about the older tier of ore) then smelt them into bars and put them at 200 or even 300% profit margins.
Only thing you need to be careful, is the amount of ores it takes to make a bar, because as i said some of the bars require 2 ores to make a single bar, so open your eyes.
Best market you want to get your greedy paws into, is the Vanilla and TBC markets, purely for the fact that people that powerlevel their professions will be picking up their bars as already smelted. But if you don't want to deal with the more slow moving bars then you might as well try to get into the Cataclysm markets, but be warned those investments could be tricky to deal with purely for the fact that most of the botters are gathering the Elementium and Obsidium Ores, and in effect keeping the prices down.
This same strategy could be used in the leather markets, or the herbs/ink market, so pretty much any market in which you turn the raw materials into finished materials. Just so you don't have to manually check the prices the see if it's worth it, you can try and get your information from any of the AH gathering sites that will give you a overview of the situation of the prices.
So as a short recap, check the prices of ores against bars, check the prices of lower tier leathers against the higher ones and check the prices of the higher inks compared with the price of the lower ones.
So, like always, keep an eye out for good opportunities out there.