I imagine that would have a lot to do with your game setting.
1. Of course some form of currency or two would work. It is fun to have several different forms of currency coined by different city-states or empires or maybe even from different times and eras...something like old Latin coins surviving into Medieval times. Different types of coin seems to always add to the realism. I loved the idea of clacks, lunars, wheels and bolgs.
2. The chapter on equipment in the basic RQ book has some good stuff. I do not have the book handy as I am at work but I think this chapter has stuff on magic crystals.
3. The Arms and Equipment book has a lot of good ideas for specialty items.
3. Runes of course.
4. Fabulously crafted mundane items like drinking horns, jewelry, cool things for characters to wear etc.
5. My favorite is valuable trade items that may be heavy, but great use for barter: Urns, boxes and casks filled with stuff like dye, incense, wine, various spices like salt (preservatives), aloe or similar ingredients for healing potions, tea, nuts other non-perishable foods, tobacco, furs, paper, cocoa, rum, rice, tar, sugar, silk, iron or copper ingots, metal weapons (possibly supplying a rebellion). Books or scrolls are also incredibly high priced items as well as various parts of legendary animals. Getting rid of these items, especially if they are hot or smuggled, can produce whole adventures on its own.
I wish lists of training and spell costs would be included in the rules as in RQ II even though many worlds may use a barter system and bargain for prices. It kind of gave GM’s and players alike a general idea of the value of things.