Having read the Dying Earth books, I can't help but make the connection that Rhialto and the other magicians in his conclave treat each other in much the same way, except for one major difference: they flat-out don't trade spells or items with each other.
There are, however, cases in the Dying Earth books when wizards engage in trade for knowledge and spells. Let's look at two:
Soon he came to a long low manse of red stone backed by dark trees. As he approached the door swung open. Turjan halted in mid-stride.
"Enter!" came a voice. "Enter, Turjan of Miir!"
So Turjan wonderingly entered the manse of Pandelume. He found himself in a tapestried chamber, bare of furnishing save a single settee. No one came to greet him. A closed door stood at the opposite wall, and Turjan went to pass through, thinking perhaps it was expected of him.
"Halt, Turjan," spoke the voice. "No one may gaze on Pandelume. It is the law."
Turjan, standing in the middle of the room, spoke to his unseen host.
"This is my mission, Pandelume," he said. "For some time I have been striving to create humanity in my vats. Yet always I fail, from ignorance of the agent that binds and orders the patterns. This master-matrix must be known to you; therefore I come to you for guidance."
"Willingly will I aid you," said Pandelume. "There is, however, another aspect involved. The universe is methodized by symmetry and balance; in every aspect of existence is this equipose observed. Consequently, even in the trivial scope of our dealings, this equivalence must be maintained, thus and thus. I agree to assist you; in return, you perform a service of equal value for me. When you have completed this small work, I will instruct and guide you to your complete satisfaction."
"What may this service be?" inquired Turjan.
"A man lives in the land of Ascolais, not far from your Castle Miir. About his neck hangs an amulet of carved blue stone. This you must take from him and bring to me."
Turjan considered a moment.
"Very well," he said. "I will do what I can. Who is the man?"
Pandelume answered in a soft voice.
"Prince Kandive the Golden."
"Ah," exclaimed Turjan ruefully, "you have gone to no pains to make my task a pleasant one… But I will fulfill your requirement as best I can."
"Good," said Pandelume…In this passage, Turjan approaches Pandelume seeking training so that he can successfully grow humans in vats and Pandelume agrees to train Turjan to his "complete satisfaction" (and will in fact do so later in the story) in exchange for Turjan stealing an amulet from Prince Kandive. The encounter is guarded – Pandelume won't even let Turjan see him – and business-like, but Pandelume, in my judgement, isn't asking Turjan for something out of proportion with the training Pandelume is willing to give; I think Pandelume is serious when he talks about "equivalence," and isn't just describing a bad deal for Turjan in flowery terms, as typical as that would be of most Dying Earth characters. Pandelume equips Turjan for his quest and Turjan accomplishes it with little trouble. There's certainly some quid-pro-quo going on here, but no outright exploitation.
Let's look at the next passage:
Prince Kandive the Golden spoke earnestly to his nephew Ulan Dhor.
"It must be understood that the expansion of craft and the new lore will be shared between us."
Ulan Dhor, a slender young man, pale of skin, with the blackest of hair, eyes, and eyebrows, smiled ruefully. "But it is I who journey the forgotten water, I who must beat down the sea-demons with my oar."
Kandive leaned back into his cushions and tapped his nose with a ferrule of carved jade.
"And it is I who make the venture possible. Further, I am already an accomplished wizard; the increment of lore will merely enhance my craft. You, not even a novice, will gain such knowledge as to rank you among the magicians of Ascolais. This is a far cry from your present ineffectual status. Seen in this light, my gain is small, yours is great."
Ulan Dhor grimaced. "True enough, though I dispute the word 'ineffectual'. I know Phandaal's Critique of the Chill, I am reckoned a master of the sword, ranked among the Eight Delaphasians as a…"
"Pah!" sneered Kandive. "The vapid mannerisms of pale people, using up their lives. Mincing murder, extravagant debauchery, while Earth passes its last hours, and none of you have ventured a mile from Kaiin."
Ulan Dhor held his tongue, reflecting that Prince Kandive the Golden was not known to scorn the pleasures of wine, couch, or table; and that his farthest known sally from the domed palace had taken him to his carven barge on the River Scaum.
Kandive, appeased by Ulan Dhor's silence, brought forward an ivory box. "Thus and so. If we are agreed, I will invest you with knowledge."
Ulan Dhor nodded. "We are agreed."And later, once Ulan Dhor's adventure is complete:
"Quiet, girl, quiet," admonished Ulan Dhor. "We are safe; we are forever done of the cursed city."
She quieted; presently: "Where do we go now?"
Ulan Dhor's eyes roved about the air-car with doubt and calculation. "There will be no magic for Kandive. However, I will have a great tale to tell him, and he may be satisfied… He will surely want the air-car. But I will contrive, I will contrive…"This is a little more confrontational, but still the agreement is to share the spoils equally. When Ulan Dhor's expedition across the sea doesn't turn up magic that is easily shareable, then Ulan Dhor begins to scheme as to how he will keep what he wants, namely the air-car, but, from the way he dealt with his Uncle Kandive earlier, this scheming seems likely to be planning ways to convince Kandive to let him keep the car, rather than outright cheating him.
I think it's important to note that when wizards in the Dying Earth trade with each other they certainly push hard bargains and aim to acquire what they desire, but that they also strike reasonably fair bargains. The most greedy, scruple-less wizards, like Rhialto and company – the ones I can't imagine striking fair bargains with each other – tend not to trade knowledge or spells or magic with each other… probably because the risk of being had is so high!
Rhialto and his "buddies" also seem to be much higher-level than Turjan and Ulan Dhor. While Turjan depends on Pandelume for transportation and Ulan Dhor sails across the ocean in a boat, Rhialto's conclave is able to traipse about the galaxy. Turjan and Ulan Dhor are much closer to the traditional idea of what a Magic-User in D&D is, both in power level and because they engage in deals with other Magic-Users for knowledge.
Why, then, didn't Gary allow for hard-bargained but fair deals for Magic-Users trading spells? Why do the PCs always get the short end of the stick when dealing with NPC Magic-Users?
I think it's probably due to two things. The first is that Gary stresses over and over again in the DMG that it's important to relieve the PCs of their wealth, and this is just another way to do that. The second is that Gary developed an adversarial DMing style that he included in the DMG and this is just one more way to stick it to the PCs. I don't think that this is necessarily a bad thing on Gary's part; from what I understand, he had power-gaming players that practically forced Gary to play in this way if he didn't want himself and his campaign world to be walked all over by his players. Clearly, both Gary and his players enjoyed playing this way - otherwise D&D would never have been published - so I don't have a problem with it, but it's also important that I take Gary's advice that worked so well for him in his DMing situation with a grain of salt if I'm in a completely different DMing situation (which I am, since I've never had power-gamers of the caliber of Gary's at my table).
I certainly don't think the avaricious nature of Magic-Users in general needs to be changed that much; after all, it's quite flavorful and, I think, adds a lot to the setting. I do think it does need to be moderated and tweaked some, and that's what I'll be doing, among other things, in some posts that I plan to begin posting soon.