It is hard to avoid allowing treasure to become just a means to an end - namely, of course, advancement. I have experimented in the past with having antiques, objets de vertu, incense, clothing, honey and the like as treasure, and the logistics of trying to get these objects out of a dungeon can be an interesting little challenge in itself. But what I'd really like to do is to try to get away from coinage (not entirely, of course) and towards an understanding of 'treasure' as more frequently comprising unique or near-unique objects which are often only really recognisable to particular types of collector, and which it takes something of an effort to sell.
Put more simply, I like the idea of PCs finding (say) a rare vase by a known designer or from a known tradition of pottery, and getting a comparatively large sum for it if they can succeed in what I will call 'The Quest to Sell' - that is, the mission of finding one of the eccentric and possibly secretive collectors of the type of treasure in question.
Jack Vance is the master at suggesting the existence of these large, dispersed, and largely unknown networks of collectors and connoisseurs operating in the background of his imaginary worlds. Take this passage from Ecce and Old Earth:
The Way of the Ten Pentalogues ran beside the Bartolo Seppi Canal, and was lined with bistros, cafes, flower stalls, booths selling fried clams and potatoes in paper packets. Along the sidestreets dim little shops dealt in specialty merchandise: curios, off-world artifacts, incunabula; rare weapons and musical instruments pitched in every key imaginable. Certain shops specialised in puzzles, cryptography, inscriptions in unknown languages; others sold coins, glass insects, autographs, minerals mined from the substance of dead stars. Still other shops purveyed softer stuff: dolls costumed in the styles of many times and places, also dolls cleverly programmed to perform acts which were polite and acts not at all polite. Spice shops vended condiments and scents, oils and esters, of an interesting sort; confectionaries sold cakes and bonbons available nowhere else on Earth, as well as dried fruits, syrops and glazes. A variety of shops displayed models of ships, ancient trains and automobiles; while others specialised in models of spaceships.
The idea of the PCs coming back from an adventure laden with rare puzzles, dolls, glass insects and a rare weapon and having to go in search of a buyer is a delightful one, not least because it opens up an underlying world of additional adventure hooks. Vance goes on a few pages later, after his main character, Wayness, has entered a shop selling 'tanglets':
Wayness turned to look at the glowing green buckles, or clasps - whatever they were - on display in the window, each on a small pedestal covered with black velvet. Each was similar but notably different from all the rest.
"They are beautiful little things; jade, I suppose?"
"Nephrite, to be exact. Jadeite gives a different feel: somewhat more coarse. These are cold and unctuous, like green butter."
"What are they used for?"
"I use them to sell to collectors," said Alvina. "All authentic tanglets are antiques, and very valuable, since the only new tanglets are counterfeit."
"What were they originally?"
"At first they were hairclasps, worn by the warriors of a far world. When a warrior killed an enemy he took the clasp and wore it on the scalp rope of his hair. In this way tanglets became trophies. The tanglets of a hero are even more; they are talismans. There are hundreds of distinctions and qualities and special terms, which make the subject rather fascinating, when you acquire some of the lore. Only a finite number are authentic tanglets, despite the efforts of counterfeiters, and each one is annotated and named and attributed. All are valuable, but the great ones are literally priceless. A hero's rope of six tanglets is so full of mana it almost sparkles. I must take extraordinary care; a single touch sours the sheen and curdles the mana."
"Poof!" said Wayness. "Who would know the difference?"
"An expert: that's who, and on the instant. I could tell you stories for hours on end." Alvina looked toward the ceiling. "I'll tell you just one, about a famous tanglet: Twelve Kanaw. A collector named Jadoukh Ibrasil had coveted Twelve Kanaw for many years, and finally, after complicated negotiations, took possession of Twelve Kanaw. On the same night, his beautiful spouse Dilre Lagoum saw the tanglet and innocently wore it in her hair to a fete. Jadoukh Ibrasil joined his wife, complimented her upon her beauty, then noticed the tanglet in her hair. Witnesses say that he turned white as a sheet. He knew at once what he must do. Courteously he took Dilre Lagoum's arm and led her into the garden and cut her throat among the hydrangeas. Then he stabbed himself. The story is usually heard only among collectors. The general feeling is that Jadoukh Ibrasil did what he had to do, and at this point the talk becomes metaphysical. What do you think?"
"I don't know," said Wayness cautiously. "It may be that all collectors are mad."
Apart from the charm of this story and the sense it communicates of a deeply layered and inhabited universe in which there are far more things than it is possible for one mind to contemplate, let alone catalogue, it also fills one's head with ideas - the PCs as tanglet thieves, the PCs as tanglet counterfeiters, the PCs digging for tanglets, the PCs searching for Twelve Kanaw. The act, in other words, of looking for somebody to sell treasure too itself becomes a rich vein of potential further adventure. This is the kind of thing that makes a campaign run itself: PCs find tanglet; PCs search for somebody to sell it to; PCs eventually discover that there are tanglet collectors; PCs sell tanglet; PCs go looking for more; NPCs try to steal tanglets from the PCs...and so on and so on for session after session after session.
Another writer expert in this kind of thing, these dark hints of hidden webs of collectors, traders, counterfeiters, burglars and fraudsters operating beneath the surface of polite society and occasionally glimpsed by those in the 'surface world' is William Gibson: I'm thinking in particular here of the subplot of Pattern Recognition suggesting a global network of people who collect Curta calculators, of course. I'm sure such people exist. I doubt many of them are eccentric billionaires engaged in a complex cold war of assassinations, burglaries and heists against one another, but it is nice to imagine that they are.