I like to pretend that humans bring gallons of blood and ichor with them to trade to dwarfs because of three key ideas:
(1) Humans simply do not understand dwarven behavior and society. They bring these bodily fluids because they actually think dwarves really like and use them for something. Why? Well, because of idea number
(2) Dwarven forts are sually covered in blood from sieges/ambushes/FUN. Traders see this everywhere and assume that the dwarves put it there on purpose, as a stylistic choice. "But wait," you might say, brimming with skepticism, "Humans bring this to me on their first trip through, and my forts aren't ALWAYS covered in gore at that point." The human traders are here, chosen for the trading trip by Human leaders, because they've traded with other dwarves before. Probably your host civilization, too. "But wait," you rudely interrupt again, ashamed of the obvious ease with which your previous point was so masterfully dismantled, "fellow dwarves have brought this sort of stuff before. Why would they be so ignorant of dwarven culture?" Which brings us to point number
(3) As your fortress moves farther and farther away, socially, from the Mountainhome, tastes change to meet the local area's flora and fauna. The Mountainhome guesses wildly each year what things might entice it's subjects living abroad in the hopes of appealing to everyone in the kingdom. They assume that your local population may have gained a taste for blood, so they ever so helpfully send you some every year.
Now for my question: what sorts of explanations do you impose for the game's various quirks and nuances?