Three Dungeon "Shops" - Playing With Treasure For XP

Put these about halfway through a large dungeon - like floor 5 out of 10, or a vestibule between two realms of a megadungeon.

This assumes you're using treasure for XP, redeemed when the players get it back to the surface.

Yvalta the Anchorite

Heir to a great house after having her siblings assassinated, Yvalta was terrified by a prophecy that greed would kill her. Unable to control her cupidity, she had herself immured in the wall of a tomb, to shield herself from the temptations of the world.

The outside of Yvalta's cell is a bas relief carved in the tomb wall, in the shape of a woman. There's a hole where one eye should be, and a hole at the shoulder of one arm.

There's a pit in the center of the room. It is bottomless. If you're feeling cheeky, there's a wall carving that reads "DEPOSITUM CUSTODI" with an arrow pointing downward.

When she has visitors, Yvalta sticks her eye to the eye hole, and her arm out the arm hole. Both are those of a lovely, though slightly soft bodied woman. She makes an archaic hand gesture of greeting. 

Her voice is muffled behind the stones.

"Hello there. You've come to plunder these tombs, then? I haven't got anything you'd be interested in. I gave all that up a long time ago" 

And if the adventurers are loaded with treasure,

"Oh please, won't you cast it into the pit? I can see it's weighing on you. It's a sin, and it's so heavy. It hurts me to look at"

And if they refuse

"Just one coin. Just a copper. Trust me. You'll see. You'll feel so much better."

Treasure cast into the hole is awarded to the players at its full value in XP, but is lost forever, and obviously cannot be spent as money.

(The prophecy will come true one day, when a greedy adventurer pries open the wall in search of treasure, killing Yvalta. Inside the cell, only her hand and face have skin, muscles or organs. The rest of her is a skeleton, which falls to the floor lifeless if the chamber is opened)

Sousa the Scavenger

An escaped slave of the cave elves, Sousa wanders the underworld, gathering the remains of adventurers killed by dungeon hazards. He is assisted by his brawny porter and lifelong friend, the cave troll Sauza.

Sousa's shop is a converted crypt, brightly lit by various enchanted objects dangling from ceiling and wall mounts. The corpses of adventurers and other dungeon inhabitants are laid out on slabs or propped up in sarcophagi (belonging to the chamber's former inhabitants). The smell of soap and candles isn't enough to obscure the stench of the bodies.

The corpses are all equipped with adventuring gear appropriate to their class. Dead fighters have mail, plate, swords, shields, etc. Wizards have spellbooks, flasks of oil, et al. Thieves have dungeoneering gear to check for traps. Roll for potions, scrolls, wands and magic items to taste. All for sale at very reasonable prices (90% of cash value)

Sousa will accept treasure as payment at full cash value, but only if it hasn't been redeemed for XP yet. Anything that's been cashed in already has the "stink of the surface" on it.

Sousa is afraid of the dark. He uses cave elf idioms, referring to surface dwellers as "daybacks". He hates elves and dwarves, but will still sell to them. His Seax Breathline is mystically drawn to the enemy's throat with every strike - a dangerous and powerful enchantment until the foe realizes the trick, at which point it's easily blocked. His cloaker leather coat makes him invisible in complete darkness, even to creatures with darkvision.

Sauza is smart, for a troll. He escaped the cave elves together with Sousa, and is violently protective of him. He knows a little earth magic, and can bore through solid rock in a hurry for quick entry or escape from dangerous situations.

If a character dies in the dungeon, and the rest of the group doesn't recover the body, it ends up in Sousa's shop, items for sale.

Xocho the Dhole

Xocho is a purple worm, uplifted to intelligence by eating too many magic users. His constant diet of greedy spellcasters has given him an appetite for exotic treasure. Unfortunately, eating so much indigestible metal has made him too bloated to hunt effectively, forcing him to engage in commerce for his feed.

Xocho eats magic items, statues, furniture, artwork, spellbooks, trade goods, and other items which have a cash value but are not, themselves, cash. After taking a moment to assess the item's value with his digestive enzymes, he vomits up 75% of the item's value in gems and coins in a big worm pellet.

If attacked, Xocho fights as a purple worm, plus the spellcasting abilities of a 5th level Magic User. If killed, his belly is full of priceless treasure, plus the melted leftovers of other priceless treasure, plus a lot of heavy objects coated in purple worm digestive acid.

Afterward

All three of these NPCs offer a convenient way for the players to "spend" the treasure they've found without taking it all the way back to the surface.

Yvalta turns treasure into XP without having to take it all the way to the surface, at cost of never being able to spend the treasure as money. If you aren't running a domain level game, the tradeoff probably doesn't matter after a certain point. In most games the amount of gold/silver necessary to reach level three or four is realistically more than the players can spend on potions, armor, weapons, etc unless they start scribing scrolls or doing other expensive stuff.

Sousa is the opposite of Yvalta, allowing the players to spend their gold without getting XP for it first, in order to get a better deal on essential equipment. I think this is a more difficult choice, since the marginal utility of wealth declines faster than the marginal value of experience. Levels are more valuable than just money and items. If the players weren't biting, I would either lower the price further, or make the items more interesting.

Xocho converts heavy or inconvenient items into a lesser amount of more easily carried currency, which must still be taken to the surface to be redeemed or spent. The Dungeon Master has to decide what he's willing to eat, so the players don't just shove every wall fitting or granite block they find into his mouth for a payoff.