I thought it might be fun to come up with ideas for things Charlatans might be selling on the planes or in Sigil - questionable items, potions, properties, etc. I'm looking for something more interesting than "fake portal key" unless it did something else interesting to the user. Basically these should be the types of things that take advantage of the incautious in the City's many markets.
To start us out, how about "Potion of Cranial Regeneration" - drink this potion and it will regenerate your head should it be removed from your body. The problem arises, of course, when one realizes that drinking a potion becomes quite impossible without the head attached. (The merchant most certainly does not suggest pouring it down the stump will be sufficient - after all, he doesn't want to be accused of false advertising)
Let's hear your ideas.
Course, if an illithid dined on your brains, you might still have a mouth to drink it with. But then again, it never said what kind of cranium you might be generating.
Perhaps a potion offering protection from flames. Careful reading of the fine print reveals it to be designed to protect you from old paramours rather than flames of the burning variety.
Maybe some ephemeral chainmail, whose only property is that it's, well, ephemeral.
Also, a snake oil salesman selling actual snake oil said to ward off snakes with a single application. Only problem is, it works only when applied to the snakes.
A guy selling a Sticks to Snakes scroll demonstrating the spell in person. Only problem is, he is just a very good magician, in the more conventional sense of the word, and is using the real world trick that the spell is based on.