What we could do is simply require more ores for weapons then other products. This way, shop owners would have to purchase more plastic, and calculate for themselves how much they should sell items for.
With this system, unless players want to waste tons of money on plastic and sell guns for lower prices, players will control the market.
Lets say a weapon costs 2000 plastic to create.
The FactoryOwner is purchasing ores for $1 each, and is selling plastic (by the 100s) at $200 each.
With this is mind, the shopkeeper sells his weapon with a $2000 markup, making the end product cost $6000. If you're skipping the middle man, and you own your own factory, then you're in luck as far as marketing. With this advantage, you'll be able to sell for a lot less markup.
Laborer >>FactoryOwner >>ShopKeeper >>Consumer
200 Ore >>20 Plastic Cases >>One Weapon
<<$2000 <<$4000 <<$6000
As times change, demands for ores and plastics will rise and fall, making prices vary. This requires players to purchase and invest with calculations.
Through this process, the players have made...
Consumer ; -$6000
Shopkeeper ; +$2000
Factory Owner ; +$2000
Laborer ; +$2000