They uh... market? Buy low, sell high? A lot of marketeers spend shit tons of time doing it though, it's something that you have to get into and the patience for.
You opened the SpaghettiO's, now you gotta sniff them.
Make
Sell
TC
but seriously marketers basically just make things that they'd think would do well, they sell it to people personally OR just on the forums, people buy it, and TC
You may want to look around in the Tutorials board for marketing tutorials. Here's a tutorial I just quickly found. It may be a little outdated, just like most threads in the Turorial section, but it might be worth a read. I don't really market, so I can't really tell you if the info inside is valuable or not.
Here's the Tutorials board if you were wondering. There may be other threads here that suit your interests. Check the index thread.