Discussion about economy can't be done without answering basic questions:
1) Why people buy?
2) What people buy?
3) How people get money to buy?
4) Why people sell?
5) What people sell?
6) How people get items to sell?
Ad. 1: People buy items they need or want. Not everything they want they need. Basic items are in higher demand when need is not fulfilled. Advanced items are bought when need is fulfilled and person still has expendable amount of money to buy.
Ad. 2: People buy items in the following order: basic items - advanced items.
Basic items are necessities required for survival. These are the items you need. In the in-game terms that would be things like: ammo, basic medical supplies, addictive drugs, etc. Consumables have varying degree of need. You need to bandage your bleeding wound to survive, but it's up to you when to do so. You need ammo to use your weapon or you rely on melee combat. Drugs are intoxicating you (can affect vision, sound, etc.), but without them you suffer substantial withdrawal effects (shaking hands - affect aim, gain perk requiring of the addict to buy another dose when withdrawal symptoms kick in, lowers character's charisma, etc.). Items needed for the job are also considered basic as are necessary for making a living by selected role.
Advanced items are wanted, but not needed in order to function. It can be anything. Custom clothes. Advanced weaponry. Augmentations. Everything that exceeds the basic level, something that is not needed, but is obtainable for the buyer nonetheless.
Ad. 3: People get money by various means. This should be tied to classes and their abilities. Cop will get "bonus" for each crime stopped while moving across the Night City. He also might gain money through corruption. Money is money. Job and payment might be tied to multi-player features too, to enhance the means by which the cash is gained.
Ad. 4: People want money. Or service. Or a favor. Most of all they want money to fulfill their own needs and wants.
Ad. 5: People sell whatever they can produce and whatever can be bought.
Ad. 6: If you can produce something and if it's needed or wanted it'll be bought, as long as people have money for it.
With all that in mind, should economy be player-based? No. It is supposed to have a strong focus on singleplayer. The in-game market will most likely be handled by NPCs, but player-made market can exist to compete (and bring profit to classes who can contribute, being on player-market should be an opportunity in comparison to regular market).
Now... What classes can do? Most importantly I ask about the Fixer and the Corporate. I could use some details to come up with sensible role for them within both single- and multi-player economy, although all classes should be able to offer their services and their crafts. I will provide an example:
A Fixer can work in terms of serving as a proxy for crafters on the Black Market of sorts: he buys an item from player for bigger price than it can be sold on the NPC-market and then sells it for higher price than he bought it from the maker of the item. The deal is attractive, because similar item might cost (much) more on NPC-market, although the quality of the item depends on skills of the maker. But how Fixer would operate outside the multi-player aspect? What they do in PnP to obtain goods?
