This short story by Andrew Plotkin is featured in System's Twilight, a now-free game I strongly recommend you play if you can get an old Mac emulator going.
Listen:
The wooden stall was not large, nor was it cleaner or better-kept than the dozen others in that corner of the market. But where the other stalls had bright paint and ribbons to draw an eye or a purse, this one was bare; its only decoration was a splintery shingle, bearing the words "Anything Bought."
"Just bought? Not sold?" Geret began, laughing, as he stepped inside. He faltered for a moment as he passed the threshold; it was dark inside, dimmer than a place ought to have been, with the May sun shining just beyond the open door. But a voice replied, "Yes; I only buy. I enjoy collecting, I fear. An eccentricity."
Shapes began picking themselves out of the shadows. Shelves of curios, vases, clocks and paperweights; racks of swords and shovels; piles of fabric, carpets, gowns; a bucket of nails. More; but Geret pulled his eyes away, and looked back to the tall man standing at the back of the stall. "All of these... things, you have," he said. "And you will not let any go? I would pay well for this --"
The tall man shook his head. "No." He was not as young as he first looked, Geret thought suddenly; there were lines around his eyes, or perhaps only weariness. "These things are all mine now." He looked up, his eyes meeting Geret's briefly. "Have you anything to sell me? If not, I regret... I am tired."
Geret hesitated, caught with some disquiet between leaving and asking the man what was wrong. Instead of either, he pulled a small book from his pack (Geret had come to the market in search of old books, and had found a few) and held it out.
The proprietor's face changed as he took the volume. Delight, joy, almost a laugh; but as he flipped through the pages, the light faded, and the weariness rose again. "Is it not what you're looking for?" Geret asked anxiously.
"No, no. My tastes are quite broad," said the tall man; it was almost an apology. "I will take it. Eight pennies."
It was not quite what Geret had paid, but he could not argue with that momentary flash of joy. He took the money, bowed, and left the stall.
Others in the town were not so soft-hearted, but to them the tall man offered more. He never refused an offer; even broken glass and rags would be taken, in exchange for a copper bit or two. The dim stall never seemed any more full; and if the proprietor never seemed pleased with his gains for more than a moment, well, the townfolk cared little. There was some murmuring when old Harasch vanished; but his young wife said he had left town to see to his merchanting business up-country. Perhaps she had been seen the night before, leaving the stall in the corner of the market, holding something that jingled; but surely that was mere alley-gossip. Of the new statue in the stall, a bent man carved of grey marble, no one spoke at all.
The town took more notice when a band of mercenaries set up outside the walls, and declared a legal state of siege-arbitrage in the name of the next town down the river. They had forces enough to blockade the town all season, and the treaty they proferred was not entirely unfair; the elders sighed and admitted that they would probably sign it -- after all, what else could be done?
What could be done was a midnight stroll outside the walls, and a quick talk with one of the mercenary guards. In the morning the mercenaries were gone, along with their entire camp. The only trace remaining was a single set of hoofprints leading north: the trail of a guard who had enough gold to start a new life, and a desire to erase the old one, before a noose caught up with him; or swords, or one of the other fates that often befall mercenaries who sell away their fellow troops.
It was late autumn when Elder Stail entered the dim stall. "You buy things," he said bluntly.
"Yes," said the tall man.
"Odd sorts of things, sometimes," Stail said. "People talk about you. Some won't stand within your stall's shadow."
"Their business." The proprietor seemed absorbed in the shelf he was dusting; toy pewter soldiers, tiny and perfect with their swords and armor.
"Yes. Some are right pleased with you, though. The midwife has nothing but good to say. For example."
"Is she well?"
"Quite well," said Stail. "Better than she has been in some time. People don't get over joint-sickness, did you know? It twists the bones eventually. The midwife has been lame for as long as I've known her. Except that last week, I saw her running. Running."
The tall man said nothing. Not so tall, the Elder thought suddenly; the man was bent a little, not standing quite straight. "At any rate," Stail when on, and stopped.
The proprietor waited.
"...The bleeding is worse," said Stail. "It eats at my guts. I doubt I will last the year." He stopped again.
The proprietor nodded slowly. Stail reached out, and a gold coin dropped into his hand. As it touched his skin, his eyes widened. He took a breath, began to speak, and then turned and fled.
Behind him, the man turned and limped slowly back to his seat. The delight on his face was already fading; after a moment, he grimaced, and sweat gleamed dully on his brow.
"I have nowhere else to go," said the woman.
The proprietor frowned from his chair (he usually sat, now) and coughed. "What do you wish to sell me?"
"You are a great wizard," she said again. "Cast a spell. Bring my love back to me."
"I am no wizard," he said. "I am an old man in a market stall, surrounded by too many things. You are young, and so is he. You will heal. This is no place for you." He seemed almost angry, beneath the pain in his voice.
The woman said, "All my hope is in you."
"No," the man breathed. But she had made an offer. He held out a gold coin, and she took it.
Her eyes went dead, dead as snow. She turned and left; and the splash from the stream outside was not followed by any noises of struggle.
The man in the stall frowned, and the light in his eyes had not entirely faded. Perhaps there was a way.
The two physicians stood close in the doorway, clutching their engraved staves. "You are destroying us," one said.
The proprietor did not rise; the sores on his face were barely visible in the dimness. "I do what I do."
The physicians exchanged nervous glances. "You are a demon, or worse," said the other. "People come to you and are healed. It is not possible."
The man smiled faintly. "You fear me."
"Yes!" both whispered. They did not move.
The man held out something that sparkled. It fell, as his fingers suddenly trembled, as the physicians suddenly stepped forward. Their staves rose and fell, and the light in their eyes was rage.
When they left, Geret stepped out of the moonlit trees and entered the stall. A breath went out of him, and then he was on his knees, feeling for broken bones, cleaning the old man's face.
Weary eyes flickered open. "You offer me your kindness?"
"None of that," Geret snapped. "You can't buy what has been given freely."
"I suppose not." The man smiled almost wryly. "What, then?"
Geret hesitated. "What do you want to buy?"
"What do I want? Now, there's an odd question." The man coughed, and fresh blood stained his bruised cheek. "I think you know the answer."
"An ending." Geret flicked his wrist, and a knife appeared in his hand. "I will sell it to you."
The old man's fingers were tight on his wrist. "Be very sure. You only have one."
"I am sure."
"It is well, then." The man relaxed. "I accept. For twenty pennies," he added, and a last smile touched his lips.
Geret nodded, finding the coins in his hand. He put them in his pocket, and set the knife to his own breast.
In the morning the stall was empty, all the miscellany gone. Only the shape of the proprietor was left, with its twisted limbs, new bruises, old scars, swollen belly, and the knife wound in its heart. Of Geret there was no trace; and if some spoke of a shape that had strode across the sky that night, radiant with joy and starlight and time, surely that was mere alley-gossip.