That's asking for a huge update bro. There are only baddies. If you are talking about monster drops, which are the only items people would mainly buy or would allow a free market. There already is a market, somewhat with retail shops and so on. But a free market is for player shops. More drops and so on makes rare items, depending on drop rates of course. But... It also require a lot more areas and levels to be designed. Don't think the developers would plainly just make a whole new section of the map for monsters too. There are other elements, but you get the idea.