Use only one shared currency and view the character equipment like rare,expensive art. This way, trading mundane items is an optional way to get money for rare equipment. This already works in MMORPGs (you go into crafting/auction house to trade to get enough money for some rare,expensive equipment).
The game could look like this:
1. starting phase:
- looting and buying simple, cheap equipment
- trading with cheap, very common items
2. transition phase
- you can sell items to boost your trading wealth
- you have enough wealth to equip quite useful equipment
3. final phase
- good items for your character are really expensive and rare, you need a lot of gold
- you have your own trading empire, you have really high amounts of wealth, and you need to plan careful how much money went into really expensive,rare equipement without wrecking your business
If you like to loot lot of rare equipment, just use some kind of money-sink, e.g. you need to use a smith to fit a piece of armor to your body, a blacksmith to balance a newly found hammer etc.