The PHB rules say this about trade goods on page 144:
Trade Goods. On the borderlands, many people conduct transactions through barter. Like gems and art objects, trade goods—bars of iron, bags of salt, livestock, and so on—retain their full value in the market and can be used as currency.
This simplifies things: when adventurers find a pile iron ingots or a sack of salt, they can sell or trade them for the listed price per pound given on the trade goods table on page 157 PHB. No complicated math needed.
However, if I am a merchant and trade in or buy trade goods from the adventurers, I have to store them, pay rent for my shop and storage, and spend my time reselling them, all for no profit. This is different from buying and selling other goods and equipment, where merchants normally buy at half price, and resell later at full price. I would have no apparent incentive to buy any trade goods from the adventurers.
I do understand that D&D is not an economics simulation, and that the game is not really set up to deal with this. But I dislike how this rule makes the world feel fake, I’m trying to fix this, so that merchants have a logical reason to buy trade goods from the party. I am considering to house-rule trade goods differently1:
Instead of fetching full value when sold by the adventurers, trade goods sold by the characters will only pay out 80% of their listed value.
Obviously, this would lessen the value of such goods found in published adventurers. Would this cause any problems or significant imbalances?
1 This still would not explain why trade exists. If I am a trader and buy a wagonload of linen in one town, and travel overland or sea and sell them in another town for the exact same amount of money back I had to spend on them (or even for 80% of that if one did also apply this to NPC merchants), there is no margin to justify the activity. The trader will be losing money, for feeding his oxen, paying off and maintaining the cart, paying guards, shelter and sustenance along the way, never mind levies, taxes etc., imposed by local authorities. Nobody in their right mind would do any trading. But for such off-screen economics, as a DM I can just declare that the traders can buy those goods much cheaper, close to the production price, and still sell them for 80% of the end-customer selling price to shopkeepers, while making a profit after accounting for risks and transport cost. It has no direct impact on the player characters.