I managed to get most of the mechanics done today, after 5.5 hours.

There's the dungeon area where you can pick up and drop items. The inventory shows each item's theoretical value.

And there's the market area where you can see how much money traders have and what they'll offer for each item. Traders prefer certain types of items so they'll pay you more for those than for others, so you won't always get the item's full value.

I still need to get the game flow implemented (market -> dungeon -> market) and add some basic menus, then I can start making the levels and balancing the prices.