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Most of us rolled and leveled our characters on whatever server that the game suggested for timezone and server load balancing. Some of us rolled characters when friends were playing. Blizzard categorizes servers in low, medium and high population. Population size makes a difference in how markets will perform for a gold maker. Population makeup will also have an impact on what will sell and what markets are profitable on a server. For example my server is medium to high pop launch day server. This gives me a large variety of customers to sell goods to. We have a healthy raiding and PVP scene for a smaller server. Even gear for transmogrification sells fairly well on this server.
How does one take a good look at their server and determine what markets will perform well? First take a look at the login server selection screen at a few different times of the day. This will tell you how Blizzard categorizes your sever. You can also look at the server status page (is that still available? We used to have to check on the servers in Vanilla all the time. Can't remember the last time the server was not available.)
Next take a look at the Undermine Journal. Select your server and then your faction. This will let you see your server’s auction house outside of the game. Look up a number of items and compare it to another server. What I have done is look at the price of various bind on equip items, gems and enchants between my server and a few friend’s servers. If the population size or makeup is drastically different you should see some differentiation in prices.
You can check out the official forums for your server. The population that actually uses the forums is much smaller than the population of the server, but it does provide some insight into the population.
Next do some in-game research. The easiest is to listen to trade chat, which I suggest you use a trade chat filter to block most of the immaturity. Are people forming pug Raids, are people recruiting for rated battlegrounds? Are people role-playing? Are there lots of low level guilds recruiting people who are leveling up characters? All of these will give you an idea of your server’s makeup.
Lastly experiment, put items up on the auction house and keep rough track of whether it sells quickly or not at all. This is the moment of truth for really determining your server population’s makeup.