This idea came to me while writing Spirit Warriors, the second in my Trilogy of Tzeentch based Chaos adventures (The first, No Mortal Power, is also available), and is incorporated into the treasure chart used in that adventure.

These are Trade Goods, valuables given to the players as treasure, but designed to make it all a little more interesting. Trade goods have a variable gold value, but it keeps on varying. Trade goods should have a variation of between 1 and 3 D6, and then a multiplier, eg. 2D6*25 gold. Each time the warriors enter a settlement roll the value of the trade goods, then the warriors can either sell at that value or keep it until they enter their next settlement. I guess the warriors could re-roll the value of trade goods after being in a settlement for a month, but what kind of warrior is crazy enough to hang around that long?

Trade goods are useful for treasure for monsters who have been raiding lately, or have other reasons for hoarding unusual valuables. Note that they tend to be better treasure than an equivalent amount of gold, because a warrior will wait until he gets a good price to sell. If he always sells immediately they are the same as gold. Also I hate drawing useless treasure like a Bronze Sigil Sword when I already have something better, like a Relic Blade or Sword of Stone, it would be much more interesting just to get the 150 gold or whatever the item is worth.

Here are some examples off the top of my head:
A. The warriors (Lvl 1) dfeat a band of goblins that have been lurking in a mountain pass and surviving through banditry, as well as some gold and a bit of magic they find:

        Ornate Pottery, Trade Goods worth 1D6*30 gold.
        Fine Yarn, Trade Goods worth 3D6*10 gold.

        Rare Dyes, Trade Goods worth 2D6*40 gold.
        Nipponese Currency, Trade Goods worth 2D6*35 gold or 250 gold whichever is more.

C. Growing mightier and more dangerous the (Now Lvl 6) warriors clear out a pyramid filled with Mummies and a viciously powerful Tomb King. The treasure room holds powerful magic, stacks of gold and:

        Strange Jewelry, Trade Goods worth 2D6*125 gold.
        Ancient Scrolls, Trade Goods worth 1D6*300 gold. If a 1 is ever rolled for value the scrolls are ruined and discarded.

Some trade goods follow special rules, note that in example A the 2 types of goods have very similar values, but the Pottery is much more variable in price, while the Yarn tends to sit around a set number. This because the value for Fine Yarn would be clearly set in most locations and haggling would be less effective. It also means that the players are more likely to end up getting 150+ gold for the pottery, while they may well settle for 120-130 gold for the yarn. In example B the currency is good obnly to those going to Nippon or who desire a curiosity, but it has a minimum price because of the materials it is made from, and the warrior could just settle for the weight of the silver. The scrolls in C are worth a lot but are very fragile, encouraging the warriors to sell quickly in order to get some value rather than none.