Stuff like this is sold almost exclusively to governments, and militaristic organizations, and at high volumes. A group of 5 people buying one bazooka is likely amazingly rare or it never happens.
In suburban America or most other countries, you're correct that civilians can't buy "bazookas".
But the idea that no small group anywhere can buy an RPG-7 is both not very imaginative and not very observant of the actual world we live in.
-- What are pirates off Somalia, bandits in Congo, tribal militias in Iraq, Pakistan, and Afghanistan, and foreign fighter volunteers in Iraq all carrying? AK's and RPG-7s. They got them somewhere.
Start thinking about WHERE, and you can start thinking about how a magic shop might actually work.
Admittedly, those guys are more like the villains than the heroes in most D&D campaigns, but the foreign fighters -- and "private military companies" who guard against them -- seen in many of these wars do bare some resemblance to PC adventurers.
If you start thinking and learning about societies where combat is part of everyday life -- whether now or in the past -- you can get some interesting ideas on your campaign world would function.
For anyone to put up the money, there has to be a return on the money. The sale price of magic items compared to the creation price makes this likely impossible.
In 3e and 3.5e, creation prices are 1/2 of sale prices. That's a 100% profit margin. Of course, you also have to give up XP, which does limit the size of the supply -- but it doesn't imply, as you would have it, that it would be "impossible" that anyone would make items for sale. You also don't allow for the possibility of TRADE in existing magic items, for which there's obviously both supply and demand in every campaign.
And doesn't a limited supply -- from people who are willing to burn XP for profit, or trade-in existing magic items -- make a specific inventory MORE realistic than assuming anything the PC's want can be bought?
I get it if you just don't CARE about background, or this element of background, and want to magic wand away the stuff that's not relevant to the adventure.
What I don't get it that you think a D&D world is inherently inconsistent and "impossible". I think it's quite possible -- and interesting -- to make it internally consistent and plausible.
And I don't get where your original hate for what I'd call "plausible world making" is coming from. If you don't like it, fine, but why is it bad for other people to like it?
I think that logical, deeply immersive worlds make for fun game--as a rule of thumb. But, the real world is more complicated that people care to learn, creating such a world would take advanced knowledge in a variety of fields, and it would still be flawed because not even experts can clearly explain everything much less emulate it.
I think most D&D players are intellectually curious and are quite capable of dealing with complex worlds that are very different from our lives in current day America.
I know my players are. My players include a guy who served in the Peace Corps in two countries in Africa, two military history buffs, and a veteran who was involved in the Somalia War. It's not a stretch for them to imagine a world where non-governmental fighters can buy RPG's, since two of them have actually been there.
Agreed, other players may not be able to find Somalia on a map, or know anything about real wars, or want anything resembling the real world in their campaigns . . . but that's why different campaign styles work for different groups.