While trading, the price of a full bin does not always correspond to its content. When it happens, buying or selling a full crate is at the player disadvantage. The price difference is HUGE and not caused by rounding error or by the price of the bin.
Example:
I'm selling a bin containing 170 socks. The full crate is worth ~3500¤. The 170 socks selected one by one are worth 13159¤.
In the same trade agrement, I want to buy a bin of 10 rope reed cloths. The full bin is worth ~1000¤. The 10 cloths selected one by one are worth 340¤.
So basically, selecting the items one by one is 10 times more profitable than buying/selling full crates.
It seems to depend on the content of the crate. In the same caravan, they sold the full crate of wool cloth 200¤, or 170¤ if selected one by one. Accounting for the price of the bin itself, and rounding error, there is no apparent bug here.
I'm playing on normal difficulty, although I tried changing it to hard economy but it did not change anything for the current caravan.