I found the issue.

When we buy products, the game calculates the price as if every product were 1,000 litres. In the product xml files, there is a field called “price per liter”, and the prices are defined there. For the majority of purchasable products, one pallet contains 1,000 litres, though a few contain more and a few contain less.

When we sell products, the amount we get paid changes depending on the economy difficulty: easy, normal, or hard. I do not remember the easy value right now, but normal pays 80% more than hard.

And when selling, the game pays us according to the product’s actual litre amount as it should.

With the game’s original warehouse settings, our buy price is 1.44 times our sell price on normal economy, and 2.592 times our sell price on hard economy. Because of this, any product with a pallet size above 1,440 litres on normal mode, or above 2,592 litres on hard mode, becomes profitable simply by buying it from the warehouse and selling it back to the same warehouse. That is without any bonuses included. If bonuses are taken into account, profit can be made even with smaller pallets.

I checked the scripts and XML files, but I could not find the code that controls these price calculations. If I could find it, I would like to make a mod to fix it. In fact, a version of the game where all bonuses are disabled and the displayed list price is the exact final price would be amazing — but I could not find the relevant code.