I'm thinking of starting to create themes to sell on Themeforest and i'd like to have my themes be updatable in the themes section under appearance. How is that done ? is there some repository that you have to use to submit your theme ? does it happen just by placing you theme on wordpress.org ?
This happens automatically if you use wordpress.org and push updates through SVN.
If you want to use your own repository you can as well, using the HTTP API and checking for updates, but this is to complex to answer so you will have to do some research, such as Updates for a private plugin?, worth checking out the WordPress Plugin Development book as it has a chapter covering this topic and of course Google.