One of the most used phrases on these forums are that all servers are different. If you have one major competitor who knows what they are doing then they can severely damage your profit making prospects.
Just make sure you sell what people want and be the cheapest. The more different items you can maintain selling then the bigger your profit potential. Lot of making money on the ah isnt hard, know your market, have a supply of goods and then be prepared to undercut.
People who make money tend to understand this, put the effort in and are well organised. Choosing the supposed best profession is not enough as you need to look at what makes you a better seller than all the other shyfflers or people who picked the same professions.