The Marketplace is the first port of call for you to get your character equipped. You can buy all kind of items there. The Marketplace is for items sold and bought from the system, this are always available and can always be sold here.
The Marketplace and its content depend in some degree on the tier of your character: Upgrading your char to the next tier unlocks new and better weapons and armors.
Let's take an example:
You've reached lvl 6 and you use the 5 class point to upgrade to the archer class. This unlocks a new bow in the Marketplace: the Nomad Bow. Then you keep playing, leveling and collecting new class points. You upgrade to tier 2: Longbowman. This unlocks another, better bow: the Khergit Bow. And it goes on like this. Upgrading to tier 3 (Heavy Longbowman) unlocks the Long Bow, tier 4 (Sniper) unlocks the War Bow.
This is the system behind it. It's of course the same with the other classes and their specific items.
Exception: You can also buy crafting goods at the Marketplace. These ones are there from the beginning. They are only obtainable at the Marketplace!
It's generally possible to sell every item at the Marketplace, but it's not advisable to do that.
Selling your items can be done through clicking on the tab “Sell your Items”, listed on the left side under the different item categories. All the stuff that you got on this character will show up with a picture, stats and the gold you would get for selling the item to the Marketplace.
You'll sooner or later sell most of the Marketplace Equipment again, as it's the way of improvement to replace it with either better Marketplace equipment or craftable stuff or even legendary items. But don't be shocked: You won't get the full price back! (A War Bow costs 12446 Dinare. Selling it to the Marketplace gives you 8713 Dinare.)
IMPORTANT: Every item can be sold at the Marketplace! So be warned: There are several stories of people having sold valuable items at the Marketplace for kinda nothing. You shouldn't sell any loots at the Marketplace. It's much better to sell them at the Auction Hall or in the NI-forum. You could also keep them, e.g. for crafting.