Is this investment opportunity a scam?

The djinn summoned by the spell won't rebel at the time of casting, but I wonder if djinn society overall might come after the caster if they feel they are being excessively abused. Imagine if some elemental creatures researched a spell called Summon Humanoid Heroes, and your party of PCs got summoned to do the bidding for a day for some djinni or efreet. I think if it happened multiple times that the PCs might look into how to make these temporary enslavements stop. In fact I think it might actually be interesting to do this to a party once, but any more than that would abusive to my players.

I would bet that merchants who buy large amounts of high-end supplies would have items similar to what we have today: permanent magical items that detect magic (aka, bill scanners). It's a bit of an investment up front, but if you're dealing in thousands of GP a month, it's worth it to keep from being scammed. A permanent staff of detect magic, usable three times a day, would cost something like 12,000 gp; somewhat expensive, but worth it, especially since it's a one-time investment. It pays for itself the first time you try to buy 6 loads of silk, only to realize they're fake...

[quote="wmarshal"]  Potentially, couldn't this be increased to 84,000 gp per month (or more) as an 11th level caster could cast this spell each day? [/quote]

Nope. The spell description states it can't be cast more often than once a week; Even at level fourteen, with multiple spell slots available and a whole pile of scrolls, you can't cast it more often than that.

That said, I guess a really greedy mage could potentially develop very similar spells to summon marids, dao, efreet, and other hypothetical genie types, until they had enough genie flavours for a full week... But frankly, I'd welcome any mage with such a clear life goal to my campaign.