there should be something that make things cheaper if you sell a myrad of it. for example you sell hundrets of stone mugs, on some point they should fall in price. so youre forced to sell more different things.
the endprice should be focused on two new things for each civilisation:
one for the material, if you sell gold and gold but buy silver, somewhere the endprice should be higher for silber than gold.
and the second for the product like food, armor, weapon or booze.
so if you sell lots of silver goblets, the price for silber and goblets/mugs is falling, so a silver crown or a obsidian mug also fall slightly in price.
so it shouldnt be you can buy whole caravans with thousands of goods with just a myrade of clutter you wont to throw out your fort. if you sell something, on some point the price is falling just realistic.
but also this should be for each civ, if you buy all the woods from the elfes and sell it to the dwarfs, the elf price is rising, and dwarfs falling.
the priceliste of your own civilisation also affects room prices. so if you make a royal obsidian dining hall for the king, but sell obsidian stuff like hell, on some point the price for obsidian is falling and suddenly the royal dining hall is just a grand one. because the king think hes room is made out of cheap material as you can sell it like hell.