I'm sorry C., but that answer is inconsistent with your position about the ubiquity of magic.
My position on the ubiquity of magic is, however common magic is, people will have developed equally economical defenses against it and will employ them. My position is always that the PC's are never the first people to have tried things that are obvious.
The spells and items that can turn you into a competent thief are- in standard D&D meta/game design logic (things in the core books are more common than things in supplements*)
Wait... what??? That might apply as a mean across campaigns... maybe. I don't think that there are demographic implications like that explicitly. And implicitly, anything that is legal to the setting (available in a particular campaign) will perforce by an evolutionary process become as common as it is useful. This reserve feat would only be uncommon in a world where bazaars don't exist, because it's just too good in a world where they do.
And then you assume that all the other options out there will force all the bazaars to have magical defenses against them...
My assumption is that whatever magic exists, always has an equally economical counter-measure. In general, characters of equal level should be able to address the threats of equal level characters, and that societies will evolve toward that equilibrium.
Honestly, how many casters in your game worlds want to be thieves?
Depends on how profitable being a thief is. Maybe the question should be, "How many thieves in your game world will want to be casters?"
Fourth, there are better ways to steal from a bazaar with magic. An illusory or summoned monster will clear the place with a stampede of humanity, leaving it ripe for a good old grab & run. And for bigger & and better stuff, I might add.
Won't work. The secret of being a thief is to stay off people's radar. This an attempt to conquer a market, not steal from it. Again you have the problem of casting in a public place, and if you can pull it off, then you have the problem that not everyone will run. Some people will pass their saving throw, or some people will take up arms to defend their property. In fact, based on my experiences with the third world, most people will - quite unlike first world people - consider their property and livelihood to be their life, and will fight to protect their possessions, because they know that if deprived of property they and their families will be destitute, devastated, and may die horrible deaths. You try to pull of a stunt with a summon monster II as a 3rd level spellcaster in a market place , you're going to be in a world of hurt in a hurry as not everyone will flee, and those that do flee will bring the 'powers that be' down on top of you. And you've got 3 rounds of monsters to protect you, the worst of which is probably a 2 HD lemure that will be a threat but not an overwhelming threat even to shopkeepers and the like. At 5th level you get major image and 5 rounds of a hell hound, and now you are starting to look like a bit of a threat, but this sort of banditry is going to give you a very short life expectancy. Could you get away with the strategy of 'conquer the marketplace' as an 11th or 15th level spellcaster. Probably so. You can handle whatever the city throws at you most likely. But sooner or later, going around muscling over small cities is going to draw the attention of the settings heavy hitters.
By contrast, this feat gives you virtual impunity to shoplift from a very early point in a spellcaster's career.
The only argument that I can see that is a defense against this is to argue that NPC's and PC's a fundamentally different. That PC's are super-special individuals that can chart their own life path, that wizardry is only available to special people, and that people don't really choose their own careers but are born to them. Thus, you might argue that feats - or the ability to cast magic - are like knacks or the ability to cast magic in Harry Potter world. But if you go that route hard, it I think is going to have other equally big implications about how the world works.
