I am going to share with you a few of the markets where I have found the most success in the last few weeks. There are so many markets on both the GAH and RMAH in Diablo 3 that it is very simple to play around and find something that works for you. Since I know the Barbarian class very well, I focus mainly on gear that I would buy for my character. This strategy allows me to quickly evaluate an item and determine if I think it will turn over quickly and takes out a lot of guess work if I was playing markets for Witch Doctors or Wizards.
Most of the RMAH sales that I have had in the last few weeks have been weapons for Barbarians. Here are a few screen shots of my transactions (note that I will sometimes send the funds to my Battle.net account and sometimes to Paypal - I learned a lot about cashing out in Markco's Gold Guide and highly recommend buying it and joining the forums, which have a good group of active members that are making hundreds of dollars playing Diablo 3)
Markco has a great video on his Gold Guide about buying and selling Skorn and I took those strategies and applied them to other weapons that Barbarians use in Diablo 3. The more that I play around on the RMAH and the GAH, the more markets that I'm finding. Keep an eye out for posts in the next few weeks with other potential markets out there!