"I can give Pilar a ride back to the Material once we're done.  We shouldn't need to reunite before that."

"I have a prediction about the contents of the transactions you want us to do, but not any special rules about them, though I could guess at sensible ones."

"Swear any promising-looking Efreet groups to secrecy about literally all aspects of the remainder of your negotiation and transaction, including the payment offered and the service sought and whether or not the transaction with you concluded successfully at all, before discussing any other trades with them."

"I put four denominations of payment in your bag.  If an Efreet group wants more payment than one of the largest denomination for one full service, or more than the least denomination of payment to agree to absolute indefinite secrecy, walk out."

He finishes casting the Telepathic Bond.

"You people try for such fascinating amounts of secrecy.  It reminds me of various episodes in my own youth that I will dramatically hint at, but say nothing more about."

"If we're not meeting up again after this, I've got a question.  Is there a reason that you, Keltham, are wearing the other of the two artifact headbands that Snack Service bought from Dispater?  Because I'm a little upset about that being how things turned out, especially when I was under the impression this was important for fighting 'Mister Doomlord', and I wonder whether Lord Dispater would be happy to hear about -"

"I traded away an even more powerful artifact to get that one, to somebody who was much more clearly on your and Carissa's side about the Mister Doomlord business."

"Sevar, you want to get started on our shopping while those two flirt?"

"Sure."

Did agreeing to let Keltham use her headband actually make him likelier to end the world? Or likelier to get something better than that? - no point trying to figure that out now. 

Welcome to the City of Brass!  Everything here is made of brass and also on fire.

Complicated divine arrangements and negotiations will have ensured that you don't meet anybody from a different planet or Prime Material plane while you shop.  You may, however, run into a devil or two, maybe an inevitable or a daemon, or denizens of Earth or Air.  To meet an angel here is not unheard-of, but demons are less welcome.

It is not literally true that everything imaginable is for sale here, but they do have a lot!  You're on sharply limited time, though, and are supposed to prioritize your main shopping trip.  Still, you've got about 50,000gp of spellsilver, at new Golarion prices, or four times that amount in old Golarion prices.

With Tarnish's approval, you could also trade away a Wish diamond here, given suitable promises of secrecy - though Keltham has expressed some worry and reluctance about arming Efreet with that kind of power, or making it obvious that the diamond market was flooded from Golarion.  Keltham/Tarnish will be happier if you trade the Wish diamond to a particular noble Efreeti intending to use it themselves, who swears an oath about how they'll use the diamond and that they won't tell anyone about the transaction.

(If what is written in many books is correct, almost any noble Efreeti can cast Wish up to once per day, given a Wish diamond.)

Carissa wants her mental enhancement first. She'll shop afterwards, if it seems like a good idea once she is smarter and better and cooler.

She doesn't, actually, prefer this world where she becomes incredibly smart and powerful to the one where Keltham never comes to Golarion and she goes to Hell. But it's - something resembling a consolation prize.

Okay, let's consider the market in the City of Brass for quintuple sequences of enhancement Wishes!

Noble Efreet are not exactly rare, in the City of Brass.  The place doesn't run much in the way of censuses, but there can't realistically be less than twenty thousand noble Efreet here, all of whom can cast 1 Wish per day.

This means there are many, many more noble Efreet than there are travelers with Wish diamonds to convert to Wishes!  The process doesn't take long and doesn't exhaust a noble Efreet, either!  And if you were very naive about how markets here operate, you might reason -

The City of Brass's price of converting a Wish diamond to a Wish should collapse to a small service charge, about what you'd pay for a few minutes of a noble Efreet's time for any other service; modulo transaction costs that both parties are incentivized to keep small.

(At least, assuming there's no such thing as a 'premium Wish' that fewer noble Efreet can offer.)

Yes, if you were very naive about how markets work, you might think that!  But naturally the City of Brass has legislated rules about minimum Wish prices - which, of course, it is illegal to explicitly tell travelers!

...then obviously you're going to have more and more entrants into the Wish-diamond-to-Wish conversion market, until all of the excess profit from this artificially high regulated price has been dissipated in marketing expenses and idle labor.  You'll get noble Efreet shops advertising Wishes on every corner of the City of Brass, as they all compete to be the shop a traveler chooses, to buy this legally overpriced service that 20,000 competitors in the city could also offer.

This artificially high price will also have the further result that fewer outside purchasers will come to the City of Brass.

Thus, the net value captured by Efreet and not dissipated in competing for overpriced transactions will be less.

Why would anyone do that to themselves?!

Because the travelers have to spend more to buy Wish-castings, and if they're losing, the Efreet must be winning!

Huh.  What's the average Intelligence and Wisdom of Efreet?

12 INT and 14 WIS.  Why do you ask?

No reason.  Anyway, dath ilan accepts the City of Brass's correction: standard economics does indeed fail to describe how trades in the City of Brass work.

Standard economics, ha!  Dath ilan is just naive about how markets work in real life!

...if you consider Lawful Evil aliens with INT 12 / WIS 14 to be 'real life', then yes, standard economic models from dath ilan do not well describe what happens when such aliens in 'real life' try to operate what they imagine to be a 'market'.

Anyways, the City of Brass was saying something about its 'market' for sequences of 5 enhancement Wishes?

Right!  Now, while it's pretty rare on any given planet for somebody to assemble 5 Wish diamonds that they want to convert to a +5 abilitystat increase, that's when you're considering just one planet.  If you consider all the planets across all the planes, that have advanced to the point of anybody there having Plane Shift, that's more like 10 parties per year that come into the City of Brass with 5 Wish diamonds that they want to convert to 5 Wishes!

And this service is overpriced as a matter of regulation, so there's many groups of 5 noble Efreet vying to persuade travelers to buy 5 Wishes from them.  Correct?

Yep!  Though not to the same extent that every noble Efreeti you come across wants to sell you 1 Wish.

Are there... 5+6+6+6+3... at least 26 groups like that, in the City of Brass?

Yes, there are.

But you'd have to walk around quite a bit, passing many more shops looking to sell you one Wish conversion, and skipping any 5-noble-Efreet groups already marked as 'tried' by a nearby Arcane Mark from one of your companion groups.

You might, at some point, have to grit your teeth and start trying to pull together a new group of 5 noble Efreet who weren't already advertising that combined service, maybe get a 2-group willing to cooperate with a nearby 3-group.  Though this will be a lot more annoying, and run a much higher risk that this is externally visible.

If the natives combine information and figure out you were buying more than one quintuple series of Wishes, that is going to point a lot more interest in your direction, if anybody catches on that far.

The ideal and hoped-for outcome is that you went around visiting a lot of shops and swearing them to secrecy, and maybe the City of Brass can figure out that part; but they don't know all those negotiations concluded successfully; and the larger City just guesses that this weird party was trying to negotiate too hard, or had some weird extra secret condition that most noble Efreet groups turned down.

This is about how Carissa expected everything to work. The important thing is that they're not weirder than other things that happen around here frequently, and that they can get their delicious delicious Wishes.