so how does the market work? i know i can buy stuff there, but how do i go to other places to sell my cargo?

.
there are currently no other places to sell things in your cargo, other then selling modules in you ship screen (if you count that as different)

so what good is it to be a trader in this game? just to transport items that are produced or mined?

What exactly do you mean by trader? As for getting minerals, that will become your main source of income so yes its good.(I think this is what your were asking)

If by trader you're referring to the Merchant occupation that's available to Inergon, Sillixx, and Human, the advantage is that you have 20% more cargo hold space. This allows you to accept larger shipments of minerals for sale (this is useful), and it also gives you more flexibility to swap out ship modules, since the uninstalled modules are stored in your cargo hold (this is arguably critical).

I'd argue that the profession is misnamed, since it doesn't really increase your income, but it certainly isn't a weak occupation.

As for the actual mechanics of trading, you accept mineral shipments from your planet on the Trade screen, and then sell the minerals you don't want to hold on to (any you aren't planning on using to build with, really). The sale is done to a NPC market. There is no official setup for trading between players in this game, although there is an option to give another player minerals or artifacts, so you can arrange a trade if it's with someone you trust (i.e., you trust them to follow through on their half of the trade after you give them your half).